How to show authority – Causative Verbs

Hi Marquito,

The Present Simple vs. Past Simple form of those verbs which take the same form in both tenses (e.g. cost – cost, let – let, hit – hit) is easily differentiable in their 3rd person (only), that is, thanks to the “-(e)s” suffix. Compare:
The book cost 100 dollars. (past fact)
The book costs 100 dollars. (present fact)

In a sentence like “I hit him hard.” / “I let her do that.” you need an adverb, e.g. now and again (= from time to time, sometimes), yesterday etc., if there is no other context (the sentence isn’t a part of a narrative such as a story), which would help the listener to work out how the speaker actually meant it – past or present.

Thus, “Tom let Sarah leave the office early.” is a sentence in Past Simple (thus, let has no “-s” suffix in this example).
Your second sentence (unless you are commenting and use short phrases e.g. in sports) should be
“Tom is letting Sarah leave the office early now/today,” where NOW means “at this very moment”, “right now”.

The below examples are also correct:
“Tom lets Sarah leave the office NOW AND AGAIN (= sometimes, occasionally).”
“Tom OFTEN lets Sarah leave the office early.”
“Tom let Sarah leave the office early today,” that is, Sarah already left.