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.

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 both express a reason. The most important difference is the sentence structure. 'denn' connects two main clauses; after it, the normal word order stays the same: '..., denn ich bin müde.' (... because I am tired.) 'da' introduces a subordinate clause; the verb comes at the end: 'Da ich müde bin, ...' (Since I am tired, ...). Learners often confuse the words because both can work like 'weil' (because). A practical tip: after 'denn' comes a complete main clause; after 'da' comes a subordinate clause with the verb at the end.

Da der Zug am Morgen ausfällt, fahren wir später los.
Since the train is canceled in the morning, we are leaving later.
Ich bleibe heute zu Hause, denn ich muss für die Prüfung lernen.
I am staying home today because I have to study for the exam.