Anstataŭ ke li aĉetis aertubon, ni uzis flikaĵon.

It means instead of and introduces an alternative to what happens in the main clause.

So the structure is:

• Anstataŭ ke li ... = instead of him ...
• ni uzis flikaĵon = we used a patch

In Esperanto, anstataŭ can work in more than one way:

• before a noun: anstataŭ aertubo
• before an infinitive: anstataŭ aĉeti aertubon
• before a full clause: anstataŭ ke li aĉetu aertubon

Here it is being used with ke to introduce a full clause.

Yes, Esperanto can often use an infinitive after anstataŭ:

• Anstataŭ aĉeti aertubon, ni uzis flikaĵon.

But a full clause with ke is useful when you want to state the subject explicitly.

In your sentence:

• li is the subject of aĉetis
• ni is the subject of uzis

If you use just the infinitive aĉeti, the subject is usually understood from context, often the same as the main clause subject. So ke helps make it clear that the buyer is li, not ni.

Often, no.

aĉetis means bought and presents the buying as an actual past event. But after anstataŭ ke, the idea is often an unrealized alternative: something that would have happened, or was considered, but did not happen.

So many speakers would prefer the -u form:

• Anstataŭ ke li aĉetu aertubon, ni uzis flikaĵon.

That sounds more like instead of having him buy an inner tube, we used a patch.

So the given sentence is understandable, but if the buying never happened, aĉetis may sound off. A learner should definitely notice this.

Because they are direct objects.

• li aĉetis aertubon = he bought an inner tube
• ni uzis flikaĵon = we used a patch

In Esperanto, direct objects usually take the accusative ending -n.

Base forms:

• aertubo
• flikaĵo

Object forms:

• aertubon
• flikaĵon

It is a compound word:

• aer = air
• tubo = tube

So aertubo literally means air-tube, which in normal English is inner tube.

Esperanto often forms compounds this way, and the last part is the main idea. So aertubo is a kind of tube.

It comes from:

• fliki = to patch, mend
• -aĵ- = a concrete thing or result related to the root
• -o = noun ending

So flikaĵo means something like:

• a patch
• a repaired piece
• a thing used for patching

In this context, it means a repair patch for the inner tube.

Because Esperanto has no separate word for a/an.

So:

• aertubon can mean an inner tube
• flikaĵon can mean a patch

You use la only when you mean the.

So if you wanted definite nouns, you could say:

• la aertubon
• la flikaĵon

But without la, the sentence is perfectly normal.

Yes. That is one reason to use a full clause.

In this sentence:

• li is the subject inside the anstataŭ ke clause
• ni is the subject of the main clause

That is grammatically possible. In fact, using ke helps make the subject change clear.

If the same subject did both actions, Esperanto would often prefer the simpler infinitive structure instead. For example:

• Anstataŭ aĉeti aertubon, ni uzis flikaĵon.

So the full clause is especially useful when the subject is not the same.