Ajusté el manillar y el sillín.

Ajusté is the first-person singular preterite of ajustar.

So it means I adjusted in the sense of a completed action in the past.

• ajusto = I adjust / I am adjusting
• ajusté = I adjusted

In this sentence, the speaker is talking about one finished action: they adjusted the handlebar and the saddle.

Because Spanish often omits subject pronouns when they are not needed.

The ending -é already tells you the subject is I. So ajusté by itself means I adjusted.

You would add yo only for emphasis, contrast, or clarity, for example:

• Yo ajusté el manillar, no Pedro.
  I adjusted the handlebar, not Pedro.

Without that kind of emphasis, leaving out yo is the normal choice.

They show where the stress goes.

• ajusté is stressed on the last syllable: a-jus-TÉ
• sillín is stressed on the last syllable: si-LLÍN

The accent on ajusté is also part of the standard spelling of many first-person preterite forms of -ar verbs:

• hablé
• miré
• ajusté

Without the accent, ajuste would be a different word/form.

Because Spanish and English package this idea differently.

In English, we usually say handlebars, which is grammatically plural.
In Spanish, the normal word is el manillar, which is grammatically singular.

So:

• el manillar = the handlebars

This is very common in language learning: the same object may be singular in one language and plural in another.

Sillín is the usual specific word for a bicycle saddle/seat.

It is more specific than asiento, which is a more general word for seat.

So on a bike, sillín is the most natural word.
If you say asiento, people will probably understand you, but sillín sounds more precise and idiomatic.

Also, its gender is simply something you learn with the noun:

• el sillín

Because Spanish normally uses the article with each separate noun here.

So el manillar y el sillín sounds natural and complete: two different parts of the bike, each with its own article.

Repeating the article is especially normal when the nouns are being treated as distinct items, which is exactly what is happening in this sentence.

A rough guide is:

ah-hoos-TEH el mah-nee-YAR ee el see-YEEN

A few useful points:

• j in ajusté is the strong Spanish j sound, like the ch in Scottish loch
• ll in manillar and sillín is pronounced in most of Spain much like y
• y becomes more like ee between words here, so y el sounds like ee el

The main stresses are:

• a-jus-TÉ
• ma-ni-LLAR
• si-LLÍN

Yes, depending on context.

In Spain, he ajustado is often used for a past action that is seen as recent or still connected to the present, especially something done today.

• He ajustado el manillar y el sillín.
  I’ve adjusted the handlebar and the saddle.

But ajusté presents it as a finished past event, more like a simple narration:

• Ajusté el manillar y el sillín.
  I adjusted the handlebar and the saddle.

So both can be correct, but they do not feel exactly the same.

Because ajustaba is the imperfect, and that usually describes:

• an ongoing past action
• a habitual action
• background information

For example:

• Ajustaba el sillín cada mañana.
  I used to adjust the saddle every morning.
• Cuando llegaste, ajustaba el manillar.
  When you arrived, I was adjusting the handlebar.

In your sentence, the speaker means one completed action, so ajusté is the right tense.