No puedo pasar por la aduana si olvido el pasaporte.

Because the understood subject is yo. Poder in the present tense changes like this: puedo, puedes, puede, podemos, podéis, pueden.

So no puedo means I can’t / I’m not able to. Spanish usually drops the subject pronoun, so (yo) no puedo is completely normal.

In Spanish, basic negation is formed with no + conjugated verb.

So:

• puedo = I can
• no puedo = I can’t

That is the standard word order. You do not normally put no after the verb in a sentence like this.

Pasar por often means to go through, to pass through, or to go via something.

So pasar por la aduana is the natural way to say to go through customs.

Here, por is part of the expression. It does not translate word-for-word as English for. It is better to learn pasar por la aduana as a set phrase.

After si meaning if, Spanish normally uses the present indicative for a real possible condition.

So:

• si olvido el pasaporte = if I forget the passport / my passport

Spanish does not normally say si olvidaré here.

This is similar to English, which also says if I forget, not if I will forget.

Yes. That would also be correct.

The difference is mainly one of nuance:

• No puedo pasar... = more general or immediate
• No podré pasar... = more clearly future, I won’t be able to go through customs...

So the original sentence is fine, but no podré is also very natural if you want to stress a future situation.

Spanish often uses the definite article (el, la, los, las) where English would use a possessive like my or your, especially when the owner is obvious from context.

So olvido el pasaporte can naturally mean I forget my passport.

You can also say mi pasaporte, and that is correct too. It just sounds a little more explicit.

Yes, that is possible.

Spanish has both:

• olvidar algo = to forget something
• olvidarse de algo = to forget about something / to forget something

So:

• olvido el pasaporte
• me olvido del pasaporte

Both can work in everyday Spanish. In this sentence, olvido el pasaporte is simpler and very natural.

Because si without an accent means if.

Compare:

• si = if
• sí = yes

They are different words, so the accent matters.

Because aduana is a feminine noun, and it takes la.

Spanish only uses el instead of la with some feminine nouns that begin with a stressed a- sound, such as:

• el agua
• el águila

But aduana is stressed on dua: a-DUA-na, not on the first a. So it stays la aduana.

Yes, but that is completely normal in Spanish.

The verb forms puedo and olvido already show that the subject is yo, so Spanish usually leaves the pronoun out:

• No puedo pasar por la aduana si olvido el pasaporte.

You could add yo for emphasis:

• Yo no puedo pasar por la aduana si olvido el pasaporte.

But in most situations, leaving it out sounds more natural.