Cum tonitrus auditur, infans flens matrem quaerit.

Cum can be either:

• a preposition meaning with, followed by the ablative
• a conjunction meaning when, since, or although

Here it is a conjunction, because it introduces a whole clause: cum tonitrus auditur.

So here:

• cum = when
• not with

If it meant with, you would expect a noun in the ablative after it, not a finite verb like auditur.

Latin often uses the passive to express a general situation without naming who does the action.

So tonitrus auditur literally means:

• thunder is heard

In natural English, that often becomes:

• when thunder is heard
• or simply when thunder is heard / when there is thunder

The sentence is focusing on the event itself, not on who hears it.

Grammatically:

• auditur = is heard
• it is the 3rd person singular present passive indicative of audio

The passive ending is -tur.

In English, with an active verb, we say:

• someone hears thunder

There thunder is the object.

But in this Latin sentence the verb is passive:

• thunder is heard

In a passive sentence, the thing receiving the action becomes the subject.

So:

• tonitrus = subject
• auditur = is heard

That is why tonitrus is in the nominative.

Auditur comes from audio, audire = to hear.

Breakdown:

• audi- = present stem
• -tur = 3rd person singular passive ending

So auditur means:

• he/she/it is heard

Here the it is tonitrus.

So the clause is literally:

• when thunder is heard

Good question. Cum can indeed take either the indicative or the subjunctive, depending on meaning.

Here it takes the indicative because it is a straightforward temporal clause:

• when thunder is heard

This is a simple factual or general circumstance.

Latin often uses cum + subjunctive for clauses that are more:

• circumstantial
• causal
• concessive
• backgrounded in a more literary way

But here the sentence is simple and direct, so cum + indicative is natural.

Flens is a present active participle from fleo, flere = to cry / weep.

It agrees with infans and describes the child:

• infans flens = the crying child
• or the child, crying
• or the child who is crying

So it works a bit like an adjective, but it comes from a verb.

You can think of the sentence as:

• When thunder is heard, the crying child looks for his/her mother.

Because Latin can use a participle where English often uses:

• an adjective
• a relative clause
• or a second finite verb

So instead of saying:

• the child who is crying seeks his/her mother

Latin can compress that into:

• infans flens matrem quaerit

This is very common in Latin. Participles let Latin say a lot in very few words.

Flens agrees with infans in:

• number: singular
• case: nominative
• gender: whatever gender infans is in context

Both are nominative singular here, because infans is the subject of quaerit.

A useful point: infans can refer to a male or female child, and flens has the same nominative singular form for masculine and feminine here, so the child’s gender is not specified by this sentence.

Because matrem is the direct object of quaerit.

The verb quaero takes an object in the accusative:

• matrem quaerit = he/she seeks his/her mother

So:

• mater = nominative, mother as subject
• matrem = accusative, mother as object

Since the child is doing the seeking, the child is the subject, and the mother is the object.

Quaero, quaerere can mean different things depending on context, including:

• to seek
• to look for
• to ask
• to inquire

Here, with matrem as the object, it clearly means:

• seeks
• looks for

So matrem quaerit means:

• he/she looks for his/her mother

not:

• he/she asks mother

Quaerit is the 3rd person singular present active indicative of quaero, quaerere.

Breakdown:

• quaer- = stem
• -it = 3rd person singular present ending for this verb

So it means:

• he/she/it seeks
• he/she/it looks for

Its subject is infans.

Latin word order is much more flexible than English word order because the endings show the grammatical roles.

Here:

• infans is nominative, so it is the subject
• matrem is accusative, so it is the object
• quaerit is the verb

So Latin does not need to rely on position as much as English does.

This order is natural because:

• the subject comes first: infans
• the participle follows the noun it describes: flens
• the object comes before the verb: matrem
• the verb comes last: quaerit

That last-point placement of the verb is especially common in Latin prose.

Because Latin has no articles.

So a noun like infans can mean:

• a child
• the child

and matrem can mean:

• a mother
• the mother
• his/her mother

The exact sense comes from context.

In this sentence, English naturally uses:

• the crying child
• his/her mother

But Latin does not need separate words for those ideas.

Not necessarily. Infans originally means someone not speaking, and it can refer to:

• an infant
• a very young child
• sometimes simply a child, depending on context

So in this sentence, infans is best understood as:

• child
• or infant, if the context suggests a very small child

English translation depends on context more than on grammar here.