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.

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.
from / of
'von' and 'seit' can both name a starting point in time. 'von' is often used with an endpoint, especially in 'von ... bis' (from ... to): the period is limited. 'seit' is used when something began in the past and still applies now. Learners confuse the words because both can name a starting point. If the situation is still continuing, 'seit' is usually correct. If you name the beginning and the end, 'von ... bis' fits.

Ich wohne seit drei Jahren in dieser Straße.
I have lived on this street for three years.
Jana ist von Montag bis Mittwoch im Kurs.
Jana is in the course from Monday to Wednesday.