Meaning of couldn't in English. short form of could not: I couldn't find my keys this morning.

How can I write could not?

couldn't. the usual way of saying or writing 'could not'. This is not used in formal writing.

What does Smth mean?

SMTH is short for something. It's one of many words that have been shortened from texting, similar to brb for “be right back” and ty for “thank you.”