can't, cant

Can't or can't is a contraction  ( short, informal form ) of  "can not"--
Mom says we can't go to the movies.
I can't stay late tonight.

Cant or cant ( rhymes with can't ) is a form of slang or secret language that belongs to a particular group--
She tried to make the story seem authentic by adding bits of the cant spoken by criminals.
Rhyming slang is one kind of cant.
Cant also means insincere rot, such as a memorized bit of philosophy one does not believe--
Surely he doesn't believe his own cant!
I'm tired of listening to this cant.

Now that you know that you can say--
"His speech is so full of thieves' cant that I can't understand it."

Note--"cant" also means on a slant or at an angle--
The cant of the wall made it difficult to hang a picture.

not, knot

Not or not is used to express a negative--a refusal, or the absence or denial of. Not may modify ( describe ) an action or a state of "being"--
It is not raining; we don't need an umbrella.
You may not go to the movies. It's too late.
It does not matter.
Do not do that.
We are not going.
We were not allowed to go.
It was not raining.
He is not sure.
"Isn't" is a contraction ( short form ) of "is not"--
It isn't raining.
"Doesn't" is a contraction of "does not"--
It doesn't matter.
"Don't" is a contraction of "do not"--
Don't do that.
"Aren't" is a contraction of "are not"--
We aren't going.
"Weren't" is a contraction of "were not"--
We weren't allowed to go.
"Wasn't" is a contraction of "was not"--
It wasn't raining.
"Won't" is a contraction of "will not"--
He won't answer.
"Can't" is a contraction of "can not"--
He can't go with us.

Knot or knot ( rhymes with not ) may be what you get in your shoelaces when they become tangled or tied in a way you didn't intend. Knots are also made on purpose to secure or fasten things with string, rope, twine, or the like--
I got a knot in my shoelace.
The girl scout learned to make a square knot.
Something is "knotted" if it has knots in it--
The strings are all knotted. I can't get them loose.
A "knotty" problem is difficult to solve--
We may never straighten out this knotty problem.

Now that you know that, you can say--
"I meant to tie a bow, not a knot."

Note-- "Ain't" is always incorrect--
I'm not going.
I am not going.
You're not going.
You are not going.
He's not going. She's not going. It's not going.
He is not going. She is not going. It is not going.
We aren't going.
We are not going.
They aren't going.
They are not going.
Remember the old rhyme--
"Ain't ain't a word, ain't ain't in the dictionary, and I ain't gonna say ain't no more."
Don't write if you find "ain't" in a dictionary, unless it fails to note that "ain't" is incorrect with an inc. or something like it.
Note # 2--some more "not" contractions --
"couldn't"  for "could not"
"wouldn't" for "would not"
"shouldn't" for "should not"
"hasn't" for "has not"
"haven't" for "have not"
"shan't" for "shall not"
"mustn't" for "must not"