Couple of tips here : when using a gerund, treat it like a noun. In the first sentence, lose the "it" since the gerund (laughing) has become the new "it"Originally Posted by mritunjay
Always contract where you can. "Can not" automatically becomes "can't" unless extreme emphasis is being placed on it. "Our prices are lowest! We will not be undersold!"
In the second sentence, "laughing" is no longer a gerund; it's now a verb that needs to be conjugated. At this very moment the tense you need escapes me, but basically it's just "laugh". I want to say it's subjunctive but it's not. Oh well.
"Choose" implies conscious decision; free will. If you're trying to say you have no control over it then "choose" is not the word you want to be using as it points to something quite the opposite. Using "help" in the first sentence was a step in the right direction, you can use that in the second sentence also.
What's all this drivel mean? It means I would write your sentences as : "I can't help laughing" and "I can't help but laugh" in the present, "I couldn't help laughing" and "I couldn't help but laugh" in the past, etc etc etc.