Cum tonitrus auditur, mater infantem flentem suscipit.

Yes. In this sentence, cum means when and introduces a temporal clause:

Cum tonitrus auditur = When thunder is heard

A learner may have seen cum with other meanings too, such as since, although, or because, but here the context and the indicative verb auditur make the straightforward temporal meaning the most natural.

Auditur is the 3rd person singular present passive of audio, meaning is heard.

So:

• tonitrus auditur = thunder is heard

Latin often uses the passive when the person doing the hearing is not important or not stated. English can do the same, though we might also naturally translate it more actively as when people hear thunder or simply when thunder is heard.

The sentence does not say who hears it. The focus is on the event itself.

Tonitrus is nominative singular here because it is the subject of auditur.

Even though auditur is passive, it still needs a grammatical subject:

• tonitrus = the thing that is heard
• auditur = is heard

So tonitrus is nominative because it is the subject of the verb.

Because flentem describes infantem, it must agree with it in case, number, and gender.

• infantem = accusative singular
• flentem = accusative singular
• both are describing the crying infant

More specifically:

• infantem is the direct object of suscipit
• flentem is a present participle modifying infantem

So the phrase means:

• infantem flentem = the crying infant or the infant who is crying

Flentem is the present participle of fleo, to weep or to cry.

A present participle usually has an active, ongoing sense:

• flens = crying, weeping
• flentem = the accusative singular form, used here to match infantem

So Latin uses a participle where English might say:

• the crying baby
• the infant who is crying

This is a very common Latin way of packing descriptive action into a noun phrase.

Because Latin has no articles.

So:

• mater can mean mother, a mother, or the mother
• infantem can mean an infant, the infant, or just infant

English must choose an article, but Latin usually leaves that to context.

So the sentence could be understood as:

• the mother picks up the crying infant
• or a mother picks up her crying infant

depending on the context in which the sentence appears.

Suscipit is from suscipio, which can mean things like:

• take up
• pick up
• receive
• undertake

In this sentence, the most natural sense is picks up or takes up in her arms.

So:

• mater infantem flentem suscipit = the mother picks up the crying infant

The verb is 3rd person singular present active.

Latin word order is much more flexible than English word order because Latin uses case endings to show grammatical function.

Here the sentence is:

• Cum tonitrus auditur, mater infantem flentem suscipit.

A very literal order would be:

• When thunder is heard, mother the crying infant picks up.

That sounds odd in English, but in Latin it is perfectly normal because:

• mater is nominative, so it is the subject
• infantem is accusative, so it is the object
• flentem agrees with infantem
• suscipit comes at the end, which is very common in Latin

Latin often puts the verb last, though not always.

Both verbs are present tense:

• auditur = is heard
• suscipit = picks up

Latin often uses the present tense for:

• a general or repeated situation
• vivid narration
• a simple scene being described

So the sentence can mean something like:

• Whenever thunder is heard, the mother picks up the crying infant
• or simply When thunder is heard, the mother picks up the crying infant

The exact nuance depends on context.

Yes. Cum is very often followed by the subjunctive, especially in clauses meaning since, although, or in certain past narrative clauses.

But here we have:

• cum ... auditur

with the indicative auditur, because this is a simple temporal statement: when thunder is heard.

So this is one of the more basic uses of cum: cum + indicative for a straightforward when clause.

Yes. Both are possible.

The participle flentem can be rendered in English in a few natural ways:

• the crying infant
• the infant who is crying
• the infant, crying

In this sentence, the crying infant is probably the smoothest English phrasing, but grammatically the Latin simply means the infant who is crying.