Can we say “the sheet comes off the corner” or “the sheet comes off in the corner”? Thanks.

Either one but “came” in the past tense!

~~in~~

How about "at the corner"? Specifying the location here.

My bedsheet is always coming off at the corner. 'The bedsheet' doesn't make sense, add ownership. Bedsheets only come off at the corner. However this is a fitted bedsheet, it has elastic at the corner. Now make it a conversation. My bedsheet is always coming off at the corner. I think I should buy some new ones.

“comes off *of* the corner” or “comes off *at* the corner” (with slightly different connotations) but not “comes off in the corner”. I’m not exactly sure why, but “in” makes the corner into, like…an area or region that can contain something, somehow, not a singular location that can have a sheet holding onto it. (Examples of typical usage for “in”: “he stood in the corner of the room during the party”, “kids like to draw the sun in the corner of the paper”, “the maximize button is in the top right corner”)

They both sound fine to me. I like off the corner slightly better.

The corner is fine, but only use in for a position inside a corner, so in the corner of a room. For outside corners such as a street corner use at. He stood at the corner of the building . He sat at the corner of the bed.

The sheet comes off the corners (if we are speaking in present tense).

Both. "The corner" is obviously the mattress. In the first case, "corner" is the direct object (receiver of the action). With the second case -- where you could also use *of the corner* -- you supplied a preposition phrase which serves as the object. Someone else here wrote to say *in* should be used for inside corners (e.g. a room), and *at* for outside corners. I could get behind that. Using *comes* vs *came* would convey whether this is currently happening on a regular basis, or if it happened once.

The “in” in the second example is redundant

“The sheet comes off at the corner” is fine but I would probably make the corner the subject to emphasize it: “The corner of the sheet always comes off the bed”