Он молчал, ибо не знал ответа.

Because the sentence describes a situation in the past: He was silent, for he did not know the answer.

Both verbs are past tense singular masculine forms:

• молчал = was silent / kept silent
• знал = knew

They agree with он (he), so they take masculine singular past forms.

In Russian, past tense verbs are commonly formed with -л plus gender/number endings:

• masculine: usually just -л → молчал, знал
• feminine: -ла → молчала, знала
• neuter: -ло → молчало, знало
• plural: -ли → молчали, знали

So молчал and знал are masculine because the subject is он.

Ибо means for, because, or since in a rather formal, literary, or elevated style.

So:

• Он молчал, ибо не знал ответа. = He was silent, for he did not know the answer.

In everyday modern Russian, people are much more likely to say:

• Он молчал, потому что не знал ответа.

So a learner should recognize ибо, but not expect to use it much in casual conversation.

Both can mean because, but they differ in style.

• ибо = literary, formal, bookish, sometimes old-fashioned
• потому что = neutral, common, everyday

Compare:

• Он молчал, ибо не знал ответа.
  This sounds more written or literary.

• Он молчал, потому что не знал ответа.
  This sounds natural in ordinary speech.

Also, ибо is often shorter and stylistically more compact.

Because after не знал Russian usually uses the genitive case for the thing not known.

• ответ = nominative
• ответа = genitive singular

So:

• знал ответ = knew the answer
• не знал ответа = did not know the answer

This is a very common pattern in Russian: negation often causes a direct object to appear in the genitive instead of the accusative.

In standard Russian, не знал ответа is the expected and most natural version here.

With negation, Russian often prefers the genitive:

• не знал ответа
• не видел дома
• не получал письма

In some contexts, accusative with negation is possible in Russian, but with знать in this sentence, ответа is the normal choice.

Russian has no articles like a or the.

So ответа can mean:

• an answer
• the answer

The exact meaning depends on context. In this sentence, English naturally translates it as the answer, but Russian does not need a separate word for that.

Yes, молчал means was silent, kept silent, or said nothing.

The verb is молчать = to be silent / to keep quiet.

In context:

• Он молчал can suggest that he stayed quiet, did not respond, or did not say anything.

So it is slightly broader than just a physical state of silence. It often implies a meaningful silence: he did not speak.

Because in Russian, many past-tense meanings that use was in English are expressed with a single lexical verb.

• молчать = to be silent / keep silent
• past masculine: молчал

So Он молчал already means He was silent or He kept silent. You do not need был here.

Using был with молчал would be incorrect.

Yes. In this sentence, ибо introduces a clause giving the reason, so a comma is normally used:

• Он молчал, ибо не знал ответа.

This is similar to how Russian often separates clauses with a comma before conjunctions meaning because, when, if, and so on.

Both verbs are imperfective:

• молчать → молчал
• знать → знал

That makes sense because the sentence describes a state or ongoing situation in the past:

• he was silent
• he did not know the answer

Perfective would not fit naturally here, because there is no idea of a completed single action like fell silent at one moment or came to know.

Yes, Russian word order is flexible, though the neutral version here is:

• Он молчал, ибо не знал ответа.

You might also see different orders for emphasis, for example:

• Ибо не знал ответа, он молчал.
• Ответа он не знал, ибо молчал would change the meaning and sound odd in context.

Russian word order often reflects emphasis, topic, and style more than strict grammatical necessity. But the original order is the most straightforward and natural for this sentence.

No. Even though English sometimes connects genitive-like forms with of, that is not how it works here.

не знал ответа simply means did not know the answer.

The genitive is required by Russian grammar in this negative construction; it does not add the meaning of in translation.

Yes, in principle it can, especially in literary style:

• Ибо он не знал ответа, он молчал.

But this sounds marked and rhetorical. The original sentence is more natural:

• Он молчал, ибо не знал ответа.

Beginning with ибо often feels more dramatic or written, and less like ordinary speech.