Duvarda portre var.

The suffix -da is the locative case marker, showing “where” something is.
• duvar = “wall”
• duvar-da = “on the wall” or “at the wall”

Turkish has an indefinite article bir, but it’s optional when you’re just stating existence.
• Duvarda portre var. = “There is a portrait on the wall.”
• Duvarda bir portre var. = “There is a (single) portrait on the wall.”
Adding bir can add a slight emphasis on “one,” but most speakers drop it in simple existential sentences.

var is the existential verb “there is/are.”
• It doesn’t change for person or tense in the same way as regular verbs.
• Its negative counterpart is yok (“there isn’t/aren’t”).
• You cannot say ben varım or o vardır—you just use var for “exists” and yok for “doesn’t exist.”

Replace var with its negative form yok:
Duvarda portre yok.

Add the question particle mı/mi/mu/mü (matching the last vowel of the preceding word) or just rely on intonation:
Duvarda portre var mı?
You can also say it with rising intonation alone: Duvarda portre var?

Turkish is typically Subject-Object-Verb (SOV). In existential sentences the order becomes Location-(Thing)-Verb. So you’ll usually see:
1) locative phrase (duvarda)
2) the noun that exists (portre)
3) the existential verb (var)

Add the plural suffix -ler/-lar to portre:
Duvarda portreler var. = “There are portraits on the wall.”
Note that Turkish sometimes omits the plural suffix in existential sentences if you’re not emphasizing the exact number.

Each word in Turkish generally has its own stress, often on the last syllable:
• duvarDA
• porTRE
• VAR
When you speak the whole sentence your main stress usually falls on var, because it’s the final word in the clause.