Eu não sei como usar o aplicativo do banco.

Yes. In Brazilian Portuguese, subject pronouns are often optional because the verb form already shows the subject.

So:

• Eu não sei como usar o aplicativo do banco.
• Não sei como usar o aplicativo do banco.

Both are natural.

Including eu can make the subject a little clearer or more emphatic, especially in speech.

In Portuguese, não normally goes directly before the verb it negates.

So:

• não sei = do not know

This is different from English, where we need do not know. Portuguese does not need an extra helper verb like do in this sentence.

Because saber is irregular.

The eu form in the present tense is:

• eu sei = I know

Some common present-tense forms of saber are:

• eu sei
• você sabe
• ele/ela sabe
• nós sabemos
• vocês sabem
• eles/elas sabem

So this is something you just need to learn as an irregular form.

Como usar literally means how to use.

Portuguese often uses:

• question word + infinitive

So:

• como usar = how to use
• onde comprar = where to buy
• quando sair = when to leave

This structure is very common and works much like English.

O is the masculine singular definite article, meaning the.

So:

• o aplicativo = the app / the application

Portuguese uses definite articles more often than English does, so in many cases where English might say just bank app, Portuguese prefers o aplicativo do banco.

Because de + o contracts to do.

This is one of the most important Portuguese contractions:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

So:

• o aplicativo do banco = the bank’s app / the app from the bank

You should think of do as a normal required form, not an optional shortcut.

It can correspond to several English expressions depending on context.

Here, o aplicativo do banco most naturally means:

• the bank’s app
• the banking app
• the app from the bank

Portuguese often uses de structures where English uses a possessive or an adjective.

Yes. In Brazil, app is extremely common in everyday speech.

So these are both natural:

• Eu não sei como usar o aplicativo do banco.
• Eu não sei como usar o app do banco.

Aplicativo is a bit more formal or neutral. App is more casual and very common in conversation.

Yes, and it changes the nuance a little.

• Eu não sei como usar o aplicativo do banco.
  = I don’t know how to use the bank app.
  This emphasizes not knowing the method or process.

• Eu não sei usar o aplicativo do banco.
  = I don’t know how to use the bank app / I can’t use the bank app properly.
  This is also natural and often means almost the same thing.

In real life, the difference is usually small.

Não has a nasal sound, which is one of the main pronunciation challenges for English speakers.

A rough guide:

• não sounds somewhat like nowng, but nasalized and without a full final g
• the ã is nasal, meaning air comes out through the nose as well as the mouth

Important point:

• it is not pronounced like plain nao
• the tilde ~ shows nasalization

If you want to sound more natural, focus on making the vowel nasal rather than trying to pronounce each letter separately.

Banco is roughly pronounced BAHN-coo, but with Portuguese sounds.

A few details:

• ban- has a nasal vowel before n
• the co is usually a clear ko sound
• stress is on the first syllable: BAN-co

In many Brazilian accents, the a in ban is affected by the nasal consonant, so it does not sound exactly like an English ban.

Because this sentence uses a very common Portuguese structure that matches English quite closely:

• Eu = I
• não sei = don’t know
• como usar = how to use
• o aplicativo do banco = the bank app

So even though Portuguese and English are different languages, some sentence patterns line up very neatly. This is one reason sentences like this are relatively beginner-friendly.

Yes, it is completely natural.

But in everyday Brazilian Portuguese, you might also hear:

• Não sei usar o app do banco.
• Não sei mexer no aplicativo do banco.
• Não consigo usar o aplicativo do banco.

The differences are:

• usar = use
• mexer em/no = handle, operate, figure out how to work with
• não consigo = I can’t manage / I’m unable to

All of these are common, but your original sentence is perfectly natural and correct.