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Nuala's grammar explanation: For and since:

We use for and since to talk about how long something has been happening:
Example 1: for five years
Example 2: since they opened in building in 1987
Example 3: for six months
We use for when we mention a period of time. For example:
Examples
for a long time for a second for two minutes
for years for ages for three days
It is possible to leave out the word for. For example:
"I've lived here for seven years" is the same as saying "I've lived here seven years." Both
are perfectly correct.
But in negative sentences, we almost always use for:
"She hasn't seen him six months" is wrong. Instead we'd say "She hasn't seen him for six
months."
And with expressions like all morning, all my life or all day, we never use for:
"He's lived there all his life" not "He's lived there for all his life."
We use since when we mention a point in time in the past, when something began. For
example:
Examples
since 2003 since last January since 4 o'clock
since July since breakfast time since the war
So, to recap. We use for and since to talk about how long something has been happening.
We use for with a period of time and we use since with a point in time.
That's all from me, good luck with your grammar challenge!
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