Derretimos la mantequilla.

It comes from the infinitive derretir, which means to melt.

So:

• derretir = to melt
• derretimos = we melt or we melted, depending on context

Because derretir is a stem-changing verb, but the stem change does not happen in the nosotros form of the present tense.

You will often see:

• derrito
• derrites
• derrite
• derretimos
• derretís (Spain)
• derriten

So derretimos is the correct we form in the present.

Yes. By itself, derretimos is ambiguous.

It can be:

• present: we melt
• preterite: we melted

Only context tells you which one is meant.

For example:

• Cada mañana derretimos la mantequilla. = Every morning we melt the butter.
• Ayer derretimos la mantequilla. = Yesterday we melted the butter.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

Here, -imos tells you the subject is we.

So both are correct:

• Derretimos la mantequilla.
• Nosotros derretimos la mantequilla.

The version without nosotros is more natural in most situations.

Because Spanish usually uses an article with nouns where English sometimes does not.

Here, la mantequilla means the butter.
The article la matches mantequilla, which is a feminine singular noun.

In Spanish, using the article here is completely normal.

Yes. Mantequilla is a feminine noun, so it takes:

• la mantequilla
• una mantequilla

And adjectives must match:

• la mantequilla fría
• la mantequilla derretida

Roughly:

deh-rreh-TEE-mos la man-teh-KEE-ya

A few useful points:

• The rr in derretimos is a rolled or trilled r.
• The ll in mantequilla is usually pronounced like y in most of Spain.
• The stress is on -ti- in derretimos and on -qui- in mantequilla.

Yes.

• Derretimos la mantequilla can mean we melt the butter or sometimes we are melting the butter, depending on context.
• Estamos derritiendo la mantequilla specifically emphasizes that the action is happening right now.

So if you want to make the ongoing action very clear, the second version is more explicit.

The first sentence is transitive: someone does the action to something.

• Derretimos la mantequilla = We melt the butter.

The second sentence focuses on what happens to the butter itself:

• La mantequilla se derrite = The butter melts / is melting.

So:

• derretimos = we cause it to melt
• se derrite = it melts

You would use the direct object pronoun la:

• La derretimos. = We melt it / We melted it.

In Spanish, object pronouns usually go before a conjugated verb.

In Spain, mantequilla is the normal word for butter.

Manteca usually refers to lard or another kind of animal fat, not ordinary butter.

So for standard butter, mantequilla is the right choice in Spain.

The neutral, normal order is:

• Derretimos la mantequilla.

You can also say:

• Nosotros derretimos la mantequilla.

If you move la mantequilla to the front, it usually needs special emphasis or a pronoun:

• La mantequilla la derretimos nosotros.

So for a basic sentence, Derretimos la mantequilla is the most natural order.