Lubriqué el piñón oxidado con grasa antes de poner el candado.

Because lubricar is a verb ending in -car, and in the yo form of the preterite, Spanish changes c to qu before é to keep the hard k sound.

So:

• lubricar → lubriqué
• just like buscar → busqué
• practicar → practiqué

If you wrote lubricé, it would sound like lubrisé, which is not the pronunciation Spanish wants here.

Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is. Lubriqué clearly means I lubricated, so yo is unnecessary unless you want emphasis, contrast, or extra clarity.

For example:

• Lubriqué el piñón... = normal
• Yo lubriqué el piñón... = I lubricated it, not someone else

Because oxidado has to agree with piñón, and piñón is masculine singular.

So the adjective must also be masculine singular:

• el piñón oxidado
• not oxidada because that would be feminine
• not oxidados because that would be plural

This is standard adjective agreement in Spanish.

In Spanish, descriptive adjectives usually come after the noun. So el piñón oxidado is the normal, neutral order.

You can sometimes put an adjective before the noun, but that usually sounds more literary, emotional, or stylistically marked. El oxidado piñón is possible, but much less neutral in everyday language.

In this context, piñón is a mechanical part: usually pinion, gear, or sprocket, depending on the machine.

Context matters, because piñón can mean different things in other situations. For example, it can also mean pine nut. But with lubriqué, oxidado, and grasa, the mechanical meaning is clearly intended.

Con is the normal preposition for the material or means used to do something. So lubriqué el piñón con grasa means I lubricated the gear using grease.

Also, grasa specifically means grease, not just any lubricant. If it were oil, you would normally say con aceite.

After antes de, Spanish uses the infinitive when the subject is the same as the subject of the main verb.

Here, the implied subject is the same person throughout:

• Lubriqué... antes de poner... = I lubricated... before putting...

If the subject changes, Spanish uses antes de que + subjunctive:

• Lubriqué el piñón antes de que él pusiera el candado.

So antes de poner is correct here because the same person does both actions.

Yes, it sounds natural. In Spanish, poner is often used in a broad way meaning to put on, to place, or to attach something. So poner el candado means to put the padlock on or to lock it with the padlock.

You could also hear things like:

• cerrar con candado = to lock with a padlock
• echar el candado = to put the padlock on, often a bit more idiomatic

But poner el candado is completely normal and clear.

El candado suggests a specific or known padlock: the one the speaker has in mind, the usual one, or one already understood from context.

If you said un candado, it would sound more like a padlock, any padlock, some padlock.

So the difference is:

• el candado = the padlock
• un candado = a padlock

Yes. Engrasé el piñón oxidado... would also be correct and often sounds very natural, especially since the sentence already mentions grasa.

The difference is mainly nuance:

• lubricar = to lubricate, more general and slightly more technical
• engrasar = to grease, more specific

So lubriqué... con grasa is correct, but engrasé may sound a bit more direct if grease is the lubricant.

A useful approximation is:

• piñón ≈ pee-NYON
• lubriqué ≈ loo-bree-KEH

A few key points:

• ñ is like the ny sound in canyon
• the accent mark in piñón shows the stress is on the last syllable
• the accent mark in lubriqué shows the stress is also on the last syllable

So the stress is:

• pi-ÑÓN
• lu-bri-QUÉ