No usamos la freidora hasta que el aceite esté bien caliente.

Esté is the present subjunctive of estar.

After hasta que, Spanish normally uses the subjunctive when the action in that clause has not happened yet and is still pending from the speaker’s point of view. Here, the oil is not hot yet, so the sentence talks about waiting for that point:

• hasta que el aceite esté bien caliente

If you used está, it would sound as if you were treating the oil’s being hot as a straightforward fact rather than something still awaited.

Spanish uses estar for a temporary condition or state, and the temperature of the oil is a temporary state:

• el aceite está caliente

That is why esté is used here.

Using ser caliente would usually sound wrong in this context, because ser is for more inherent or defining characteristics, not for the oil’s current temperature.

Spanish often uses the present tense for:

• general procedures
• instructions
• habits
• rules

So No usamos la freidora... can sound like a general instruction: We don’t use the fryer until...

It is not necessarily talking about what is happening right this second. It can mean something like This is how we do it or This is the rule.

If you wanted a more direct command in Spain to a group, you could say:

• No uséis la freidora hasta que el aceite esté bien caliente.

Yes. With nosotros forms of -ar verbs, the present and the preterite can look identical:

• usamos = we use or we used

Context tells you which one it is.

In this sentence, it is understood as present, because:

• the sentence has a general-instruction feel
• esté points to a not-yet-completed situation

So here it means we use / we do not use, not we used.

That is how standard negation works in Spanish: no goes directly before the conjugated verb.

• No usamos... = We do not use...

Unlike English, Spanish does not need a helper verb like do. So you do not say something equivalent to we do not do use. Spanish simply uses:

• no + verb

Spanish uses the definite article more often than English does.

Here:

• la freidora refers to the fryer being talked about in the situation
• el aceite refers to the oil involved

In English, you might sometimes say the fryer and the oil, but English also drops articles more often in general statements. Spanish is more likely to keep them.

So this sounds natural:

• No usamos la freidora hasta que el aceite esté bien caliente.

not because the objects are super-specific in a narrow sense, but because Spanish commonly uses the article with nouns like these.

Bien here is an intensifier. It does not literally mean well in the usual English sense.

So bien caliente means something like:

• properly hot
• good and hot
• nice and hot
• very hot

It suggests the oil must be hot enough, not just slightly warm.

No. It depends on how the action is viewed.

Use the subjunctive when the action is still pending or anticipated:

• No usamos la freidora hasta que el aceite esté bien caliente.

Use the indicative when the action is presented as a completed fact:

• Ayer no usamos la freidora hasta que el aceite estuvo bien caliente.

So the key idea is not hasta que by itself, but whether the event is still unrealised or already treated as factual.

Because hasta que introduces a full clause with its own subject and verb:

• el aceite = subject
• esté = verb

So the structure is:

• hasta que + subject + verb

Without que, you would not have a normal finite clause.

Not naturally in this sentence.

The infinitive structure hasta + infinitive works best when the subject is the same as the main clause. For example, if we were the ones becoming ready, an infinitive could work.

But here, the thing becoming hot is the oil, not we. So Spanish naturally uses a full clause:

• hasta que el aceite esté bien caliente

That makes the subject clear.

It could be, depending on the situation.

• No usamos la freidora... sounds like a general rule, procedure, or explanation.
• No uséis la freidora... is a direct instruction to more than one person in Spain.

So both can be natural, but they do different jobs:

• No usamos... = We don’t use... / This is the rule
• No uséis... = Don’t use... addressed directly to a group

Because you specified Spain, no uséis is the normal informal plural command there.