Culex in cubiculum intrat, et mater fenestram statim claudit.

Because culex is in the nominative case, which is the case usually used for the subject of a sentence in Latin.

Here, culex means mosquito (or gnat), and it is the thing doing the action of intrat.

So:

• culex = subject
• intrat = enters

Because in can mean different things depending on the case that follows it.

Here, cubiculum is in the accusative case, so in cubiculum means into the bedroom/room and shows motion toward a place.

A very important contrast is:

• in + accusative = into / motion toward
• in + ablative = in / inside / location

So:

• in cubiculum intrat = he/she/it enters into the room
• but in cubiculo est would mean he/she/it is in the room

Cubiculum is the accusative singular form of a second-declension neuter noun.

Its basic form is cubiculum, cubiculi.

It is accusative here because it follows in with the idea of movement into a place.

A learner may notice that cubiculum looks the same as the nominative singular. That is normal for many neuter nouns:

• nominative singular: cubiculum
• accusative singular: cubiculum

For neuter nouns, nominative and accusative are often identical.

Intrat is:

• third person singular
• present tense
• active voice
• indicative mood

It comes from intrare, meaning to enter.

So intrat means:

• he enters
• she enters
• it enters

In this sentence, the subject is culex, so the natural English sense is the mosquito enters.

Because Latin has no articles like English the or a/an.

So a noun like culex can mean:

• a mosquito
• the mosquito

and mater can mean:

• a mother
• the mother

You decide which is best from the context. That is very normal in Latin.

Because mater is the nominative singular form, and it is the subject of the second clause.

It is a third-declension noun, and third-declension nouns do not all follow the same visible pattern as first- or second-declension nouns. The dictionary form is:

• mater, matris = mother

Here it stays mater because it is the one doing the action of claudit.

If it were the direct object, you would expect matrem instead.

Because fenestram is the direct object of claudit, so it must be in the accusative case.

The basic noun is:

• fenestra, fenestrae = window

Its forms here are:

• nominative singular: fenestra
• accusative singular: fenestram

So:

• fenestra = the window as subject
• fenestram = the window as object

In this sentence, the mother is doing the closing, and the window is what gets closed, so Latin uses fenestram.

Claudit is:

• third person singular
• present tense
• active voice
• indicative mood

It comes from claudere, meaning to close.

So mater fenestram statim claudit means the mother closes the window immediately.

A useful thing to notice is that both intrat and claudit are third person singular present forms, matching their subjects:

• culex intrat
• mater claudit

Statim is an adverb, meaning immediately or at once.

It modifies the verb claudit and tells you how quickly the mother closes the window.

Latin adverbs often have fairly flexible placement, so statim could appear in different positions without changing the basic meaning very much. Here it comes before claudit, which is completely natural.

Because Latin word order is more flexible than English word order.

English relies heavily on position:

• The mother closes the window
  not
• The window closes the mother

Latin relies much more on case endings to show who is doing what.

So in mater fenestram statim claudit:

• mater is nominative, so it is the subject
• fenestram is accusative, so it is the object

That means Latin can move words around more freely for emphasis, rhythm, or style. For example, a Roman could also say:

• mater statim fenestram claudit
• fenestram mater statim claudit

and the endings would still show the grammatical roles.

Because Latin verbs usually already include the subject information.

Both intrat and claudit are third person singular, so the verb itself already tells you:

• he
• she
• or it

That is why Latin often leaves out subject pronouns unless they are needed for emphasis or contrast.

So Latin does not need to say:

• ea claudit = she closes

unless it wants to emphasize she in particular.

Et is the normal Latin word for and.

It connects the two clauses:

• Culex in cubiculum intrat
• mater fenestram statim claudit

So the sentence describes two linked actions:

1. the mosquito enters the room
2. the mother immediately closes the window

Yes. Both verbs are in the present tense:

• intrat = enters
• claudit = closes

Latin often uses the present tense very straightforwardly for actions happening now or in a vivid narrative style.

So the basic grammar here is simple present, even if English translation might vary a little depending on context.