363. Since for For
(X) She's lived here since two years.
(O) She's lived here for two years.
Place the preposition "for" before words or phrases denoting a period of time:
for three days, for six weeks, for two years, for a few minutes, for a long time.
Use it with any tense except the present.
Note: "For" is often omitted. We can say, I've been here for two years. "or" I've been here two years.
364. From for Since
(X) Susan's been ill from last Friday.
(O) Susan's been ill since last Friday.
Place the preposition "since" before words or phrases denoting a point in time:
since Monday, since yesterday, since eight o'clock, since Christmas.
When we use "since", the verb is usually in the present perfect tense, but it may be
in the past perfect. I was glad to see Tom. I hadn't seen him since last Christmas.
Note: "From" can also denote a point in time, but it must be followed by "to or till".
He works from eight o'clock till one o'clock without a break.
365. After for In
(X) I may be able to go after a week.
(O) I may be able to go in a week. "or" I may be able to go in a week's time.
When speaking of a period of time in the future, use "in", and not "after".
Here "in" means "after the end of ~".
366. In for Within
(X) I'll come back in an hour. - if you mean before the end of an hour.
(O) I'll come back within an hour.
"In" means "after the end of~", "within" means "before the end of ~".