Mi fidas vin, ĉar via respondo estis sincera.

Because vin is the direct object form of vi.

In Mi fidas vin, the person being trusted is the object of fidas, so Esperanto adds -n:

• vi = you
• vin = you as the object

So:

• Mi fidas vin = I trust you

A native English speaker may not expect this, because English usually does not mark nouns this clearly, but Esperanto does it very regularly.

It can mean either.

Esperanto vi is used for:

• singular you
• plural you
• formal you
• informal you

So vin can mean you in any of those senses, depending on context.

Because via respondo is the subject of the second clause, not the object.

In ĉar via respondo estis sincera:

• via respondo = your answer
• estis = was
• sincera = sincere

Your answer is the thing that was sincere, so it is the subject. Subjects do not take -n.

You would only use -n if that noun phrase were a direct object.

Via is the possessive form related to vi.

It means your.
So:

• vi = you
• via = your

Possessive words in Esperanto behave like adjectives, which is why via ends in -a.

Because possessives like mia, via, lia, ŝia, nia, ilia usually take the place of the article la.

So Esperanto normally says:

• via respondo = your answer

not:

• la via respondo

For an English speaker, it may help to think of via as already making the noun definite enough.

Because Esperanto adjectives always end in -a, and here the word is an adjective describing respondo.

So:

• sincera = sincere

After esti (to be), Esperanto uses an adjective to describe the subject:

• La respondo estis sincera = The answer was sincere

It would not use an adverb here, because the sentence is describing what the answer was like, not how something was done.

Because it is linked to the subject, not to a direct object.

In via respondo estis sincera, the adjective sincera describes respondo, and respondo is the subject. Since the subject has no -n, the matching adjective also has no -n.

If the noun were plural or accusative, the adjective would match that:

• sinceraj respondoj = sincere answers
• sincerajn respondojn = sincere answers as objects

Ĉar means because.

It introduces the reason:

• Mi fidas vin = I trust you
• ĉar via respondo estis sincera = because your answer was sincere

So the second part explains the reason for the first part.

Yes. Esperanto word order is fairly flexible, especially with clauses like this.

You can also say:

• Ĉar via respondo estis sincera, mi fidas vin.

That means the same thing, but it puts the reason first.

Because Esperanto marks grammatical roles clearly, it often allows more freedom than English.

Because estis is the past tense of esti.

So:

• estas = is
• estis = was

The sentence refers to the answer as something in the past, so estis is natural:

• via respondo estis sincera = your answer was sincere

If you said estas, it would mean your answer is sincere, which gives a different time sense.

Yes. They come from the same root respond-.

• respondi = to answer
• respondo = an answer

This is a very common Esperanto pattern:

• verb in -i
• noun in -o

So learners often benefit from spotting the root:

• respond- = answer/respond idea

Ĉ is pronounced like ch in church.

So ĉar sounds roughly like char.

A few useful details:

• ĉ = ch
• a is like the a in father
• r is typically rolled or tapped, though beginners are usually understood even if their r sounds more English-like

Since ĉar is only one syllable, the stress question does not really matter here.