Antaŭ ol mi sendas la leteron, mi metas ĝin en koverton kaj kontrolas la poŝtmarkon.

That is a very common question. Ol does often mean than, as in comparisons, but it also appears in the fixed expression antaŭ ol, which means before when a verb or a whole clause follows.

So:

• antaŭ la leciono = before the lesson
• antaŭ ol mi sendas la leteron = before I send the letter
• antaŭ ol sendi la leteron = before sending the letter / before I send the letter

Here, ol helps connect antaŭ to the action that follows.

The ending -as marks the present tense in Esperanto. So:

• sendas = send / am sending
• metas = put / am putting
• kontrolas = check / am checking

In this sentence, the present tense works well if the speaker is describing a usual procedure or a general sequence of actions: first I do this, then I do that.

If you wanted to talk about one specific future occasion, you could also use -os:

• Antaŭ ol mi sendos la leteron, mi metos ĝin en koverton kaj kontrolos la poŝtmarkon.

So the given sentence sounds like a routine or general statement.

Yes. That is perfectly possible.

If the subject is the same in both parts of the sentence, Esperanto often allows either:

• Antaŭ ol mi sendas la leteron...
• Antaŭ ol sendi la leteron...

The version with mi sendas states the subject explicitly. The infinitive version sendi is a little more compact.

A full clause is especially useful when you want to make the subject clear or when the subject might be different.

Ĝin means it, and here it refers to la leteron.

So the sequence is:

• la leteron = the letter
• mi metas ĝin = I put it

The base pronoun is ĝi, and the -n is added because it is the direct object of metas.

This is important in Esperanto: pronouns take the accusative ending too.

• ĝi = it
• ĝin = it as a direct object

Because this sentence shows movement into something.

In Esperanto, en can be followed by:

• no -n for location: en koverto = in an envelope
• -n for direction or movement into: en koverton = into an envelope

Here, the letter is being put into the envelope, so koverton is correct.

Compare:

• La letero estas en koverto. = The letter is in an envelope.
• Mi metas la leteron en koverton. = I put the letter into an envelope.

Because they are direct objects.

In Esperanto, the direct object normally takes the -n ending.

In this sentence:

• mi sendas la leteron = I send the letter
• mi metas ĝin = I put it
• mi kontrolas la poŝtmarkon = I check the stamp

So all three objects are marked:

• leteron
• ĝin
• poŝtmarkon

This -n ending is one of the main features that makes Esperanto sentence structure clear.

Esperanto has la for the, but it has no separate word for a or an.

So:

• la leteron = the letter
• la poŝtmarkon = the stamp
• koverton = an envelope / envelope

The sentence treats the letter and the stamp as specific, but the envelope as not specifically identified. That is why koverton appears without la.

Yes. It is normal and standard.

The first part, Antaŭ ol mi sendas la leteron, is a subordinate clause. Esperanto usually separates that kind of clause from the main clause with a comma, especially when the subordinate clause comes first.

So the punctuation here is completely natural.

Not completely. Esperanto word order is more flexible than English word order, largely because the -n ending shows the direct object.

The given order is the most neutral and natural:

• mi metas ĝin en koverton kaj kontrolas la poŝtmarkon

But other orders are possible for emphasis, for example:

• La leteron mi metas en koverton...

That said, learners should usually stick to the normal pattern first:

• subject + verb + object

That will sound the most natural in most situations.

Yes, you could.

• meti ... en = to put something into
• enmeti = to put in / insert

So these are both possible in practice:

• mi metas ĝin en koverton
• mi enmetas ĝin en koverton

The first version is very straightforward and common. The second is a little more compact because the idea of into is already built into enmetas.