La valvo lasas aeron en la aertubon.

Because aeron is the direct object of lasas.

• lasi = to let, allow
• lasas = lets / allows
• The thing being let or allowed is aeron = air

So in La valvo lasas aeron..., the valve is allowing air, and Esperanto marks that direct object with -n.

• aero = air
• aeron = air (as the direct object)

Here en shows movement into something, not just location.

Esperanto often uses -n after a preposition when there is a sense of direction toward a place.

So:

• en la aertubo = in the air tube / inside the air tube
• en la aertubon = into the air tube

In this sentence, the air is being let into the tube, so aertubon takes -n.

The difference is:

• en la aertubo = in the air tube (location)
• en la aertubon = into the air tube (motion/direction)

This is a very common Esperanto pattern. A learner should watch for it with other prepositions too when motion is involved.

For example:

• La kato estas sur la tablo = The cat is on the table.
• La kato saltas sur la tablon = The cat jumps onto the table.

So in your sentence, aertubon shows that the air goes into the tube.

lasas comes from lasi, which usually means to let, to allow, or sometimes to leave depending on context.

In this sentence, it means something like:

• lets air into the air tube
• allows air into the air tube

So La valvo lasas aeron en la aertubon means the valve permits the air to go into the tube.

Because -as is the present-tense ending in Esperanto.

• lasi = to let
• lasas = lets / is letting
• lasis = let / allowed
• lasos = will let

So La valvo lasas aeron... is in the present tense: The valve lets/allows air...

It is a compound word:

• aer- = air
• tub- = tube
• -o = noun ending
• -n = accusative / direction marker here

So:

• aertubo = air tube
• aertubon = air tube, with -n

Esperanto makes compound words very freely, and the last part is usually the main idea. So aertubo is a kind of tube, specifically an air-tube.

Because both nouns are definite:

• La valvo = the valve
• la aertubon = the air tube

In Esperanto, each noun phrase usually takes its own article if it is definite. English does the same here:

• The valve lets air into the air tube.

You would not normally use one la for both nouns.

Because air here is being used in a general or mass-noun sense, not as the specific air.

So:

• aeron = air
• la aeron = the air

In this sentence, the idea is simply that the valve lets air into the tube, not necessarily some already-identified air.

Yes. Esperanto word order is fairly flexible because endings show the grammatical roles.

The normal order here is:

• La valvo = subject
• lasas = verb
• aeron = direct object
• en la aertubon = prepositional phrase

But other orders are possible for emphasis, such as:

• Aeron la valvo lasas en la aertubon.

That said, the original order is the most natural and easiest for learners.

It is a normal Esperanto word, but like many Esperanto words it is internationally recognizable. It corresponds to words like valve in English and similar forms in other European languages.

This is common in Esperanto, especially with technical vocabulary. The word still behaves exactly like any regular Esperanto noun:

• valvo = valve
• valvoj = valves
• valvon = valve (object)
• valvoj / valvojn as needed

Yes, lasi can also mean leave, depending on context. For example:

• Mi lasis la libron sur la tablo. = I left the book on the table.

But in your sentence, that meaning does not fit. A valve does not leave air into a tube; it lets/allows air into a tube. So context makes the meaning clear.

Esperanto does not need an extra verb here. The idea of motion is already shown by:

• en
• plus the directional -n in aertubon

So lasas aeron en la aertubon already naturally means lets air into the air tube.

If you translated too literally from English, you might expect an extra verb, but Esperanto expresses the idea more compactly here.