Learn Hardcore Portuguese (Portugal): Eu tento ser sincero com os meus amigos.

You would only see tentares in certain literary or very formal constructions (an old‑fashioned infinitive form), not in this kind of simple sentence.

Do I really need to say eu? Can I just say Tento ser sincero com os meus amigos?

You can absolutely drop eu and say:

Tento ser sincero com os meus amigos.

In Portuguese, the verb ending already shows the subject (here: tento = I try), so subject pronouns are often omitted.

Using eu usually:

Adds emphasis (Eu tento ser sincero…, even if others don’t)
Or appears when you want extra clarity or contrast.

Both forms are correct and natural.

Why is it ser sincero and not estar sincero?

In Portuguese, ser is used for more permanent, characteristic, or essential qualities, while estar is used for temporary states or conditions.

Being sincero (honest, sincere) is treated as a character trait, not just a passing state, so you use ser:

Eu tento ser sincero = I try to be (as a person) honest.

Using estar sincero would sound odd and unnatural in most contexts, as if honesty were just a temporary mood.

Could I say Eu sou sincero com os meus amigos? What’s the difference from Eu tento ser sincero…?

Yes, you can say:

Eu sou sincero com os meus amigos. = I am honest with my friends.

The difference is in meaning:

Eu tento ser sincero… = I try to be honest. Maybe I’m not always successful, but I make an effort.
Eu sou sincero… = I am honest (a stronger, more confident statement about your character).

So the original sentence emphasizes the effort, not just the final quality.

Why is it com os meus amigos and not para os meus amigos?

Com means with, and it expresses the idea of being in a certain way in relation to someone:

ser sincero com alguém = to be honest with someone

Para usually means to / for, and would suggest direction, purpose, or benefit, not the relationship style.

So:

Eu tento ser sincero com os meus amigos. ✅ (natural, correct)
Eu tento ser sincero para os meus amigos. ❌ (unnatural in this meaning)

Why do we say os meus amigos instead of just meus amigos?

In European Portuguese, it is very common (and usually more natural) to use a definite article before possessives:

os meus amigos = my friends
a minha casa = my house
o meu carro = my car

In Brazilian Portuguese, people often drop the article and say simply meus amigos, minha casa, etc.

In Portugal:

os meus amigos sounds completely natural and is often preferred.
meus amigos without os can sound a bit more formal, written, or stylistic, though it’s not wrong.

So for European Portuguese, os meus amigos is the default everyday choice.

Why is it os and not as in os meus amigos?

Os is the masculine plural definite article.
Amigos here is:

Masculine
Plural

So everything must agree in gender and number:

o amigo → os amigos (masc. singular → masc. plural)
a amiga → as amigas (fem. singular → fem. plural)

If you were talking only about female friends:

as minhas amigas = my (female) friends

If the group is mixed or you don’t specify gender, os meus amigos is used.

If I’m a woman, should I say sincero or sincera?

Sincero agrees with the subject’s gender (the person who is trying to be honest).

Man speaking:
- Eu tento ser sincero com os meus amigos.
Woman speaking:
- Eu tento ser sincera com os meus amigos.

For plural subjects:

Group of men or mixed group: Nós tentamos ser sinceros…
Group of only women: Nós tentamos ser sinceras…

How do I make this sentence negative? Where do I put não?

In Portuguese, não goes directly before the conjugated verb:

Positive: Eu tento ser sincero com os meus amigos.
Negative: Eu não tento ser sincero com os meus amigos. = I don’t try to be honest with my friends.

Notice:

não comes before tento (the conjugated verb)
It does not go between tento and ser.

What’s the difference between sincero and honesto in Portuguese?

They are close in meaning but not identical:

sincero = sincere, straightforward, open about what you feel or think.
honesto = honest, morally upright, not cheating or stealing, but also honest in speech.

In the context of ser sincero com os meus amigos, both are possible, but:

sincero focuses more on being open and truthful in what you say.
honesto can sound a bit more about moral honesty in general.

So Eu tento ser sincero com os meus amigos sounds very natural when you mean you speak openly and truthfully with them.

Could I replace tento with procuro? For example: Eu procuro ser sincero com os meus amigos?

Yes, you can, and it sounds natural:

Eu procuro ser sincero com os meus amigos.

Nuance:

tentar = to try, to make an effort (emphasis on the effort)
procurar (in this sense) = to seek, to strive to (emphasis on a continuous search or aim)

They overlap, and in everyday speech they’re often interchangeable here.
Tento ser sincero feels a bit more like I make an effort to be honest,
Procuro ser sincero feels a bit more like I strive / aim to be honest.

Is there anything specifically “European Portuguese” about this sentence?

Yes, mainly the possessive with the article:

os meus amigos is the default in European Portuguese.
In Brazilian Portuguese, many speakers would more naturally say meus amigos (without os).

Apart from that, the sentence Eu tento ser sincero com os meus amigos is grammatically correct and understandable in both European and Brazilian Portuguese.