B1    We haven't eaten anything since we got here.
B2    We haven't eaten anything since we've been here
B3    I've been feeling sick (ever) since I ate that cake.

2. When the main verb in a sentence with a since clause (since you arrived) or a since phrase (since Tuesday) refers to a period of time including the present, a present perfect tense is necessary.

C1    See examples B1 - B3.
C2    I have been there since six o'clock.
C3    Since winning the prize, he's been really happy.

3. If the whole sentence refers to past time, a past perfect tense is required in the main clause. The since clause generally contains a verb in the simple past tense.

D1    We hadn't eaten anything since we arrived.
D2    I'd been feeling sick ever since I ate that cake.
D3    I hadn't eaten anything since six o'clock.

4. Since always implies time if (a) it is a preposition (examples C2, D3) or (b) it is followed by a present participle (example C3).

5. Since or for ?

Since can only be used with a moment in time - expressed either as an adverb (e.g. yesterday), an adverb phrase (e.g. the start of term) or a verb of action (e.g. I ate...). Since can never be used with a duration. We cannot say: since three hours Duration is expressed with for.
E1. I've been here since last week.
E2. I've been here for three hours.

Since implying cause:

1. If a since clause contains a present tense, since must imply cause, not time.

F1    Since it is raining, we are staying indoors.
F2    We will stay indoors since it is raining.
F3    Since he lives in Peru, he doesn't often visit us.

2. If the verb in the main clause and in the since clause are both in the preterite (simple past ) , since almost certainly implies cause, not time;

G1 I ate all the chocolates since you left them behind.
G2 Since he was poor, he never took taxis.

(To imply time, a present perfect is normally used for the main verb

G11 I've eaten all the chocolates since I got home.)

3. Since, implying cause, is synonymous with as.

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