I can take is pronounced I cun take and you stress take, like this, IcanTAKE.
I can't take, you pronounce can't as KAN and you actually say the two words separately: Ican't take.
So, using Portuguese spelling to help:
I can do it: aikãnDUit.
I can understand: aikãnãnderSTAND.
I can take it: aikãnTEIKit.
I can't do it: aiKEN DUit.
I can't understand: aiKEN nãnderSTAND.
I can't take it: aiKEN TEIkit.
The American a in can't is pronounced by opening your mouth to say the a in salto, but saying the é in fé, instead. This sound does not exist in Portuguese, that's why it's funny to hear Brazilians keep saying "Bed Boy" instead of "Bad Boy".
In British English there is no possible confusion since they say can (kaen or kun) and the a in can't is pronounced as the a in car. Kahnt, but remember the British do not pronounce R before consonants, just between vowels. That's why they usually say Lawr and order instead of law and order, they don't know where some r's are anymore, they also say, "Sandrar is". This R that doesn't exist, but is pronounced is called Intrusive R, by the way.