El alquiler es más alto del que esperaba.

In this sentence, el alquiler means the rent: the amount paid to live in a flat, house, etc.

In Spain, alquiler is the normal everyday word for rent. It can also refer more broadly to renting or a rental, depending on context, but here it clearly means the rent amount.

A useful note for Spain: renta usually does not mean rent in this context. It more often means income or appears in things like tax return.

Yes. Esperar can mean both:

• to wait
• to expect

Here it means to expect, because of the context.

So del que esperaba means something like than I expected / than the one I expected, not than I was waiting for.

This dual meaning is very common in Spanish, so learners often need to rely on context.

Because alquiler is a masculine noun in Spanish, so it takes el.

Grammatical gender in Spanish does not always match the ending of the word. Even though alquiler does not end in -o, it is still masculine.

That also affects the adjective:

• el alquiler alto
• not la alquiler alta

So in your sentence, alto is masculine singular because it agrees with alquiler.

Yes. In Spanish, alto is often used for things like:

• prices
• rent
• salaries
• costs
• temperatures
• levels

So un alquiler alto means high rent, just as English says high rent or a high price.

Here, alto does not mean physically tall. It means high in amount or level.

Yes, in many situations you could.

• más alto focuses on the level/amount
• más caro focuses on the idea of being expensive

So these are both natural:

• El alquiler es más alto de lo que esperaba.
• El alquiler es más caro de lo que esperaba.

The difference is small in many contexts. Más alto sounds a bit more like you are talking about the numerical amount; más caro sounds a bit more like your judgement that it is expensive.

Because ser is the normal verb for identifying or describing something as a general characteristic.

Here, the sentence is describing the rent as higher than expected, so es is the natural choice:

• El alquiler es alto.

With prices and costs, Spanish can sometimes use estar when talking about a temporary situation in the market, for example Los precios están muy altos últimamente. But in a basic statement about what the rent is like, ser is the standard choice.

Because Spanish often leaves out subject pronouns when they are not needed.

The verb form esperaba already gives enough information in context, and Spanish prefers to omit yo unless there is a reason to stress it.

So:

• del que esperaba
• del que yo esperaba

Both are possible, but the version without yo is more normal unless you want emphasis or contrast.

Because esperaba is the imperfect, and that fits the idea of an ongoing expectation in the past.

Here the meaning is something like:

• than I was expecting
• than I had expected

That past expectation is treated as background, not as a single completed event.

If you used esperé, it would usually sound like a completed action, and very often learners would hear it more as I waited rather than I expected. So esperaba is the natural form here.

Because de + el normally contracts to del in Spanish.

So:

• de + el = del

That is a standard rule.

For example:

• vengo del centro
• hablo del problema

The same thing happens here:

• del que esperaba

The only major exception is when El is part of a proper name, such as de El Escorial.

That is a very good question, because both patterns exist.

de lo que esperaba

This is often the more neutral and common way to say:

• than I expected

Example:

• El alquiler es más alto de lo que esperaba.

del que esperaba

Here, el que means something like the one that and refers to an understood masculine noun, such as:

• el precio
• el alquiler
• el importe

So the idea is closer to:

• The rent is higher than the amount/the one I expected.

So:

• de lo que esperaba = more directly than I expected
• del que esperaba = more like than the one/amount I expected

In everyday speech, many speakers would probably prefer de lo que esperaba in this sentence, but del que esperaba is understandable and grammatical if you interpret el que as standing for an omitted masculine noun.