The abbreviation ('cause) is slang for "because". It is therefore incorrect usage. You won't see it in written English. You may see it in comics but it is always written with the apostrophe ('cause).

'cause / 'cuz = because cause without the " ' " is the verb that was mentioned above.

Yes, your teacher is correct. Because and cause are two different words, so you cannot use one in place of another (or "prefer" one over the other). Because = subordinating conjunction We did not go on the picnic because it was raining. Cause = verb The high unemployment rate causes people to worry about their own jobs. Cause = noun The cause of the accident is unknown. There are non-standard forms that lead to confusion. NO! I bought you this cause (cuz / coz) I like you. Those are not allowed in standard English. If you write something to show how someone talks (accent, dialect, reduced speech forms, etc.) you can write "I bought you that 'cause I like you." The apostrophe shows that you know the real form is because.