
da
there
Indicates a place; at this place, here or there.
Stell die Kiste bitte da in die Ecke.
Please put the box there in the corner.
there / since / then
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there
Indicates a place; at this place, here or there.
Stell die Kiste bitte da in die Ecke.
Please put the box there in the corner.

since / because
Introduces a subordinate clause that gives a reason or cause; because, since.
Da das Wetter schlecht ist, bleiben wir zu Hause.
Since the weather is bad, we're staying at home.

then
Refers to a specific point in time or moment; then, at that moment.
Ich war gerade eingeschlafen, da klingelte das Telefon.
I had just fallen asleep when the phone rang.

there
Used to confirm the presence of a person or thing.
Ist jemand da?
Is anyone there?
You can recognize the function of 'da' from the sentence structure. If 'da' introduces a subordinate clause (meaning: because/since), the conjugated verb stands at the end. If 'da' refers to a place or point in time, it is an adverb and the verb stands in position 2.
Da er müde war, ging er früh ins Bett.
Since he was tired, he went to bed early.
When 'da' gives a reason, it is a conjunction that introduces a subordinate clause. The conjugated verb must then stand at the end of the subordinate clause.
Ich kann heute nicht kommen, da ich einen wichtigen Termin habe.
I can't come today because I have an important appointment.
As an adverb for a place ('dort' = there) or a point in time ('dann' = then), 'da' often stands at the beginning of the sentence. In this case, the verb follows directly after it (inversion).
Wir waren fast am Ziel. Da sahen wir das Ortsschild.
We were almost at our destination. Then we saw the town sign.
The meaning 'anwesend' (present/there) is often used in simple, short sentences. Typical situations are on the phone, at the door, or during an attendance check.
Hallo, ist Anna da?
Hello, is Anna there?
because
Denn and da can express the same reason. Denn connects two main clauses with normal word order, while da introduces a subordinate clause and presents the reason as more familiar or self-evident. Use denn for an independent explanation and da for an assumed reason.

Ich bleibe zu Hause, da ich krank bin.
I’m staying home because I’m sick.
Ich bleibe zu Hause, denn ich bin krank.
I’m staying home because I’m sick.