Learn Hardcore Portuguese (Brazil): Eu penso que ela acredita em você.

acho is more common in everyday spoken Brazilian Portuguese.
penso can sound a bit more careful, reflective, or formal, though it’s still normal.

In most casual conversations, Brazilians would more often say Eu acho que… than Eu penso que…, but both are correct.

Can I drop Eu and just say Penso que ela acredita em você?

Yes. Subject pronouns are often dropped in Portuguese because the verb ending shows the subject.

Eu penso que ela acredita em você.
Penso que ela acredita em você. (also correct)

Both mean I think that she believes in you. The version without Eu may sound slightly more formal or written, but it is totally correct.

Do I really need the que? Can I say Eu penso ela acredita em você?

You must use que here. It introduces a subordinate clause, like that in English:

Eu penso que ela acredita em você. = I think that she believes in you.

You cannot say:

✗ Eu penso ela acredita em você. (incorrect)

If you omit que, the sentence sounds wrong to native speakers in this structure.

Why is it acredita em você and not just acredita você?

In Portuguese, acreditar usually needs a preposition:

acreditar em alguém = to believe in someone
acreditar em algo = to believe in something

So:

Ela acredita em você. = She believes in you.

You cannot say:

✗ Ela acredita você. (incorrect)

However, when the object is a clause, you often use que:

Ela acredita que você vai conseguir. = She believes (that) you will manage.

Does acreditar em você mean “believe you” or “believe in you”? Is there a difference?

Acreditar em você usually means believe in you (have faith in you, trust your potential).

If you want to emphasize believing what you say, you can use:

Ela acredita em você. (often understood as believes in you, but can be context‑dependent)
Ela acredita no que você diz. = She believes what you say.
Ela acredita na sua versão. = She believes your version (of the story).

So:

acreditar em você → normally believe in you (support, trust, faith).
to make it clearly “believe what you say,” specify it: acreditar no que você diz.

Why is it acredita (present indicative) and not acredite (subjunctive) after penso que?

In Portuguese, after verbs like achar, pensar, acreditar (when affirming your belief), you normally use the indicative, because you are presenting the idea as something you consider true or likely:

Eu penso que ela acredita em você. (indicative → normal, neutral).

The subjunctive often appears when you deny or doubt that belief:

Eu não penso que ela acredite em você. = I don’t think that she believes in you.
Eu duvido que ela acredite em você. = I doubt that she believes in you.

So:
penso que → indicative (acredita)
não penso que / duvido que → usually subjunctive (acredite).

Could I change the word order, like Eu penso que ela em você acredita?

No, that would sound very strange and unnatural in Brazilian Portuguese.

The natural word order is:

Eu penso que ela acredita em você.

You can move parts around a little for emphasis in some sentences, but here ela acredita em você should stay in that order. Putting em você before acredita in this sentence is not idiomatic.

Can I omit ela and say Eu penso que acredita em você?

Grammatically, you can omit ela because acredita (3rd person singular) shows that the subject is he/she/it:

Eu penso que acredita em você.

However:

Without ela, the subject is ambiguous (could be he, she, or you formal).
In real conversation, Brazilians often keep ela to avoid confusion:
- Eu penso que ela acredita em você. (clear: she)

What is the difference between você and tu here? How would it look with tu?

Você and tu both mean you, but usage depends on the region and level of formality.

Your sentence with tu would typically be:

Eu penso que ela acredita em ti.

Key points:

você uses em você.
tu uses em ti (prepositional form of tu).
In most of Brazil, você is more common.
tu is common in parts of the South and Northeast, but patterns vary by region (and sometimes people mix tu with você verb forms).

How would I make this more formal, for example talking about an older person with respect?

To speak respectfully about you when talking to someone older or in a formal context, you can use o senhor (to a man) or a senhora (to a woman):

Eu penso que ela acredita no senhor. = I think she believes in you, sir.
Eu penso que ela acredita na senhora. = I think she believes in you, ma’am.

Here:

em + o senhor → no senhor
em + a senhora → na senhora

Is there a more formal or literary alternative to acredita?

Yes, you can use crer, which is a bit more formal or literary:

Eu penso que ela crê em você.

It has the same basic meaning (she believes in you), but:

acreditar is more common in everyday speech.
crer can sound more formal, religious, or literary.

Why is there no comma before que in Eu penso que ela acredita em você?

In Portuguese, you normally do not put a comma between a verb and its direct subordinate clause introduced by que:

Eu penso que ela acredita em você. (correct, no comma)
✗ Eu penso, que ela acredita em você. (incorrect in neutral prose)

So, unlike some optional commas in English, Portuguese punctuation rules are stricter here: no comma between penso and que in this structure.

How would I say I don’t think she believes in you? Does anything else change?

The direct, everyday version is:

Eu não penso que ela acredita em você.

However, in Brazilian Portuguese, when you negate verbs like achar/pensar, many speakers prefer the subjunctive in the subordinate clause:

Eu não penso que ela acredite em você.
Eu não acho que ela acredite em você. (very common)

So you may hear both, but:

With não penso/acho que, acredite (subjunctive) is very natural.

How do I pronounce Eu penso que ela acredita em você?

Approximate Brazilian pronunciation (São Paulo/Rio style), in English-friendly terms:

Eu → like “eh-oo” blended, often close to “ey-o”.
penso → PEHN-soo (first e like in pet; so not as in so, more like soo).
que → kee (in Brazil, often like “ki”).
ela → EH-la.
acredita → ah-kreh-JEE-ta (the di often sounds like soft gee).
em → nasal, like “eng” but short.
você → vo-SEH (final ê like say but shorter; c like s).

Spoken smoothly:
Eu penso que ela acredita em você. → ey-o PEHN-soo kee EH-la ah-kre-JEE-ta eng vo-SEH.

Is Eu penso que ela acredita em você the most natural way to say this in Brazilian Portuguese?

It is correct and natural, but many Brazilians would more commonly say:

Eu acho que ela acredita em você.

or, in even more casual speech, drop Eu:

Acho que ela acredita em você.

Your original sentence is fine; just be aware that acho is more frequent than penso in everyday conversation.