could
v.v. used to talk about what was possible in the past or to ask for something politely. You also use it to talk about things that might happen in the future.
v. the past tense of 'can', used to express past ability or possibility. It also functions as a modal auxiliary to express conditional possibility or to make polite requests and suggestions.
I could swim well when I was younger.
Could you please open the window to let some fresh air into the room?
The researchers suggested that the new data could potentially change how we understand climate patterns, though further verification is required.
From Middle English coude, couthe, cuthe, from Old English cūþe, past indicative and past subjunctive form of cunnan (“to be able”) (compare related cūþ, whence English couth). Cognate with German konnte, Swedish kunde. The -l- in the spelling was added in the early 16th century by analogy with should and would; this analogy formerly affected the pronunciation as well and was probably assisted by the tendency for /l/ to be lost in those words (and so not written, leading to shudd, wode, etc).
A modal verb followed by the base form of the main verb without 'to'. It does not change form for different subjects (I could, she could).
I could to goI could goModal verbs like 'could' are followed by the bare infinitive; do not use 'to' before the main verb.