Jupiter has given you some good rules. Well done, Jupiter.
(But the first sentence should be in a future tense: (1) I will definitely come late today.)
Regarding Quoc’s sentences:
(b) is correct and is the most “standard” or “typical”
(a) is also acceptable as an alternative to (b)
© sounds odd. (Is your teacher trying to stress the word “late”?)
(d) sounds odd. (Is your teacher trying to stress the word “today”?)
(e) also sounds odd, but might be possible if you place a comma after “Definitely”. The word “definitely” would then sound like an agreement with or verification of something someone else has just said.
(f) also sounds odd, but might be possible if the word “definitely” is separated from the rest of the sentence and added as an afterthought (i.e., additional punctuation necessary)
I find it hard to believe that an English teacher would tell you that you can put the word “definitely” anywhere you like and leave it at that. :?
Does that mean that when you have a test, your teacher will accept the word “definitely” anywhere in every sentence? :roll: