Since is followed by a time (e.g. 6 o’clock, yesterday), a year or date (e.g. 1945), or an event (e.g. the end of the war):

• There hasn’t been a world war since 1945.
• Things have changed a lot since the end of the war.

We use since to talk about a period from that time in the past to now:

TIMELINE

Note that we use since with the Present Perfect (e.g. have changed). But a verb after since is about an event, and it must be in the Past Simple:

• Things have changed a lot since the war ended.

We use for with a period of time (e.g. 3 months, 6 years), to answer the question How long?:

• How long did the war continue?
• It continued for 6 years.

TIMELINE

The verb in the example is Past Simple (continued), but we can use for with other tenses to talk about a period of time in the present, the past, or the future:

• A: How long is Daniel staying?
• B: He’s staying for five days.

TIMELINE

• There hasn’t been a world war for many years.
• It will probably snow for several hours.

If we talk about the start and end of a period, we use from (NOT since) and to:

• The war lasted from 1939 to

We can use ago with a Past Simple verb to say when something happened:

• A: When did the Second World War start?
• B: It started over 80 years ago. (= It started 80 years back in the past from now.)

Look at these sentences, where first means ‘for the first time’ and last means ‘for the last time’:

• Jane and Bill first met 10 years ago.
• The planet Pluto was first seen in 1940.

• A: When did you last go on holiday?
• B: I last went on holiday two years ago.