盖【gài】

盖 at the start of a clause offers the explanation for what was just said, with a touch of caution: "for", "the reason is, presumably". It is a written word, and modern formal prose uses it mostly as 盖因 or 盖由于 "this is because".

Explaining without 因

On its own, 盖 still introduces a reason, often with a closing 也 in older prose.

Opening a general statement

In classical prose, 盖 can also open a passage that lays down a general point, the way "for" or "indeed" does in old English prose.

Not the verb 盖

The everyday 盖 is a verb, "cover" or "build": 盖房子, 盖章, 盖被子. The classical word sits at the head of a clause, right after a comma, and is followed by a reason. 该 "the said" is a different character.