
seit
since
As a preposition indicating the beginning of a period of time that continues into the present.
Er wohnt seit drei Jahren in Berlin.
He has been living in Berlin for three years.
since
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since
As a preposition indicating the beginning of a period of time that continues into the present.
Er wohnt seit drei Jahren in Berlin.
He has been living in Berlin for three years.

since
As a conjunction (often as 'seitdem'), introducing a subordinate clause that indicates the point in time when the action of the main clause began.
Seit er den neuen Job hat, ist er viel glücklicher.
Since he got the new job, he has been much happier.
Whether as a preposition or conjunction, 'seit' (since/for) always describes a state or action that began in the past and continues into the present. For this reason, the main clause is almost always in the present tense.
Seit dem Umzug fühle ich mich viel wohler.
Since the move, I feel much more comfortable.
When 'seit' (since/for) is used as a preposition before a noun, it always takes the dative.
Ich lerne seit einem Jahr Deutsch.
I have been learning German for a year.
As a conjunction, 'seit' (since) introduces a subordinate clause. The conjugated verb comes at the end of this subordinate clause. In this function, 'seitdem' can often also be used.
Seit ich in Deutschland lebe, trinke ich viel mehr Tee.
Since I have been living in Germany, I drink much more tea.
for
Für refers to a limited or intended duration without presupposing a beginning in the past. Seit names the beginning or the time that has elapsed so far in a situation that still continues in the present. Für zwei Jahre therefore usually describes a plan, while seit zwei Jahren describes a state that is still ongoing.

Lena arbeitet seit zwei Jahren in Berlin.
Lena has been working in Berlin for two years.
Lena arbeitet für zwei Jahre in Berlin.
Lena is working in Berlin for two years.