1-Knowing that I haven’t taken any exercice during the day, I’ll have a light meal at night.
1a.Knowing that I haven’t taken any exercice during the day, I have a light meal at night.

2-Whenever I know that I haven’t taken any exercice during the day, I’ll have a light meal at night.

Can 1 and 1a be used instead of 2 (for a habitual action)?

Dear N:

2 is best for habitual action because of the word “whenever.” 1 and 1a are both OK, but they may refer to a single action. There is nothing to indicate single or repeated action. Context is critical for those, but 2 is clearly a repeated action. In any case, the future tense is more commonly used than the present in the main clause, so 1 would be more typical than 1a. However, because 1a is in the simple present, it could be used for a repeated action as well.