grammar - When to add "the", "a" or when not to add "the" or "a"?

What's the difference (or correct way of saying it) between the followings:

1. "Review plan" vs "Review a plan" vs "Review the plan"?




2. "Agree on plan" vs "Agree on a plan" vs "Agree on the plan"




3. "Implementation of the plans" vs "Implementation of plans? vs "Implementation of plan"?

I suppose it boils down to when do you add "the" or "a" or not add it at all?

Answer

This isn't an academic answer, but a practical one based on the English language as it is used.

Review plan - this could be a name for an existing plan designed to review something. The name of the plan is 'Review Plan'. Or, it is shorthand or note form (perhaps in a 'To Do' list)

Review a plan - referring to a plan (any plan) being reviewed

Agree on plan - shorthand, note form (wouldn't be spoken). The correct usage in normal speech or writing is one of the other two (both are equally correct)

Implementation of the plans - the correct usage in formal writing or normal speech. The other two are both note form or shorthand.