"Do" is difficult for me.

It’s not abbreviated. We use can/do interchangeably in “can/do you speak English?” however it’s not necessarily interchangeable with other verbs. Using “Can you go to the park?” is asking about the ability to go to the park. Using “Do you go to the park?” is asking if it’s a habitual action or something that has been done in the past.

In theory, any statement can also be a question. If it's written, you can obviously tell from the question mark; when spoken you can generally tell by the rising tone at the end, but that's not always very clear and obvious. As for "do", yeah it's used to ask a question which has a yes/no answer. (but never used with "to be"). It's often omitted as the tone should indicate a question is being asked.

You speak English Is both a statement and possible question with how they ask. Rising intonation is a question. You do speak English isn't a question, even with intonation, but a statement. Maybe a question if the person isn't a native speaker. I'm a linguist and a second-language acquisition teacher, so my parameters of what is and isn't acceptable is a bit different from most native speakers.

What happens here is that for some reason, lexical verbs don't have the propety of inversion (People don't say "Speak you English?"), so, to circumvent this problem English speakers must change the original statement by adding a "dummy" do (which means that it doesn't have any meaning whatsoever, its only purpose is to maintain the patterns of the language). For example, "You speak English" is converted to "You do speak English", and then you can do the invertion "Do you speak english? BTW, the use of "do" in the sentence "You do speak English" may be interpreted as an emphatic use of the verb "do".

Generally speaking, only auxiliary verbs (be, can, could, dare, do, have, may, might, must, need, ought, shall, should, will, & would) can be the main verb in a question sentence. If the verb you want to make into a question isn't one of those, you have to add (and conjugate) "do". E.g. "You speak English." "To speak" isn't an auxiliary verb, so to make it into a question it has to become "you do speak English" which is inverted to make "do you speak English?"

sometimes we do say "do." <-- that's an example right there. here's another: "do you speak english?" "i do speak english." so the "do" is optional

When you form “yes or no” questions in English, the most common way is to invert the sentence. This means you place the verb before the subject. But you can’t just do it with any verb. It has to be a modal verb or a linking verb. That is, it has to be an auxiliary verb, or a linking verb (usually a form of “be”). If it’s a modal/auxiliary verb, you place it before the subject, and the main verb stays where it was after the subject. “You can see the shadows behind the tree.” “Can you see the shadows behind the tree?” “You have seen this movie already.” “Have you seen this movie already?” “He should take this class.” “Should he take this class?” If the verb is a linking verb, the linking verb moves in front of the subject and the predicate stays where it was, after the subject. “You are sure.” “Are you sure? “She is the professor for this class.” “Is she the professor for this class?” “Charlie is bringing his girlfriend to the party.” “Is Charlie bringing his girlfriend to the party?” IF THERE IS NO AUXILIARY OR LINKING VERB, then a form of “do” is inserted as a placeholder or “dummy” auxiliary verb. “You see the stars in the sky.” “*Do* you see the stars in the sky?” “He eats meat and dairy.” “*Does* he eat meat and dairy?” “You study for 2 hours every day.” “*Do* you study for 2 hours every day?” Note that if the main verb is in past tense, you need to use “did.” When you do this, the main verb is in the bare infinitive form. “You took that class last semester.” “*Did* you *take* that class last semester?” “She saw that movie last weekend.” “*Did* she *see* that movie last weekend?” These are the general rules for inverted sentences. You may need to invert sentences for a number of reasons in English. The big 2 are yes/no questions like these, and negating (“I saw you” vs “I *didn’t see* you”). Some set phrases use inversion as well, like “not only….” Let me know if you have any more questions. Hope this helps.

This is because "do" is the verb in that sentence, and when you ask a question in English (or any Germanic) language, the subject and the verb are switched. But I don't know why the sentence isn't "speak you english".

For the question, "Do you speak English?" you can answer it in several ways. There can be small differences in meaning, and people often use any of these answers interchangeably. "Yes, I speak English." - I know how to speak English. "Yes, I can speak English." - I am capable of speaking English. This might mean that I don't necessarily use it. For example, I haven't spoken English for the last ten years, but if I need to, I can speak with people in English. "Yes, I do speak English." - I use English on a regular basis. The "do" gives the statement emphasis that this is an activity I engage in. It's stronger than the two previous statements. The three statements above are often used to mean the same thing.

Negatives and questions require an auxiliary “do”, English features “subject auxiliary inversion”, it’s worth noting words like “can, must, should, could, will” are all modal, but modals are a type of auxiliary. In english words that aren’t auxiliary verbs, like “speak” require the assistance from an auxiliary to complete negatives and questions. I do, I do not I speak, (add aux for negation) I do speak, (add negative particle) I do not speak-> (contraction) I don’t speak You speak, (add aux for question) You do speak, (add subject aux inversion), Do you speak? The words “to be” and “to have” are sometimes auxiliaries sometimes not, (ie “I am a man” not aux, “i am walking” aux…. “I have a pen” not aux, “i have eaten” aux) -To be is always inverted, and never takes an auxiliary -To have is inverted when it is paired with another verb, but not so much on its own, it also still typically requires a negative auxiliary Worth noting have is sometimes (in a bit of a posher form of english maybe) never paired with an auxiliary- like “have you a pen?” Or “I haven’t any money” (quoted from harry potter and the philosophers stone). But in spoken modern english, people tend to use an auxiliary or the word “got” (“have you got a pen?” “I haven’t got any money”)

You often see/hear “do” dropped off sentences like that, particularly in slang. It happens so much that I’d almost consider it implied most of the time

Do is used to help clue the listener into the fact that your asking a question and not making a statement. It's used in most questions where you don't use verb can (to be able to).  Do you speak English?  Do you want to go to the park? Do you want to take a walk? Do you shop at that store every day? Do you like ice cream? You can use do in your other example to show a change of opinion or to stress that someone has that skill/ability. Oh! You do speak English. (Implied that the speaker thought they didn't before) I don't know how to play chess. You do know how to play chess. I taught you last summer.  Notice in your can example that for the question you reversed the verb (can)/ noun (you) order. This is correct and indicates a question. Can you speak English? question  You can speak English. Statement    You can speak English? You might hear questions like this, but it's very casual and can be confusing if you sentence isn't rising enough to indicate a question to the listener.

It's because English requires an auxiliary verb (things like can, will, sometimes have) to form questions and do negation. So if there isn't an auxiliary verb in a sentence, 'do' is added to take the place of one. This is called 'do-support', you can read about it here: https://www.learngrammar.net/english-grammar/do-insertion-or-do-support

Under the correct conditions, you can ask the question, "You speak English?" 1. You need to end the question with a rising tone. (ESL students who speak a tonal language natively sometimes have difficulties with this.) 2. Asking "You speak English?" implies "You seem to be a person who probably would not speak English." 3. Asking questions this way may come across as insulting. **When it doubt, use the "Can you speak English?" pattern instead.** \------ Also, strictly speaking, English has only two tenses--present and past tense. Everything else typically requires multiple verbs. (Present: I speak. Past: I spoke. Future: I **will** speak.) The sentence "You speak English." is a simple present tense statement. English has a tense for that. Everything is simple. For "Do you speak English?", things get complicated. You need another verb to make a proper question--Wikipedia calls that [subject-auxiliary inversion](https://en.wikipedia.org/wiki/Subject%E2%80%93auxiliary_inversion). (There's more on this topic in [Do-support](https://en.wikipedia.org/wiki/Do-support).) You don't need to memorize the phrase "subject-auxiliary inversion". (I didn't know the phrase myself.) For now, you can get by just fine with "If it isn't simple present tense or simple past tense, it's probably going take at least two verbs to to say it in English."

I don't know if my reply will help here but in this cases, "do" and "can" are functionally synonyms but technically they are being used strictly in accordance with their exact definitions, respectively. If we want to know if someone speaks english, we want to know about their capabilities, but this also implies that we are curious about their habits. "Do you speak English" is technically a query about whether the person has a potential or actual habit of speaking English. "Can you speak English" is technically a query about whether the person has the capability of speaking English. Both "do" and "can" are fine because the answer to either variant also answers the question we are really asking: is the person an English speaker. Potentially my explanation may confuse someone but I thought it might be worth saying to point out that even though English is often illogical, in some cases-- like this one--it's completely logical.

I see 'do' as an emphasis marker. 'I did forget that' emphasizes that I forgot. 'Do you speak English?' emphasizes that I’m asking a question. 'I don’t eat carrots' emphasizes that carrots are not my thing.