En el restaurante pedimos mejillones para compartir.

Pedimos can be either:

• present tense: we ask for / we order
• preterite: we asked for / we ordered

So the form itself is ambiguous. You usually know which one it is from the context. In a sentence about what happened at a meal, it often means the preterite: we ordered.

You usually need context, because the nosotros form of pedir is the same in the present and the preterite.

For example:

• Siempre pedimos mejillones = We always order mussels → present
• Ayer pedimos mejillones = Yesterday we ordered mussels → preterite

In your sentence, En el restaurante pedimos mejillones para compartir, the most natural reading is often we ordered, but without more context, both are grammatically possible.

Because Spanish usually drops subject pronouns when the verb form already shows who is doing the action.

• pedimos already tells you it is we
• so nosotros pedimos is possible, but usually unnecessary

You would include nosotros only if you want emphasis, contrast, or clarity:

• Nosotros pedimos mejillones, pero ellos pidieron calamares.

Because en means in / at, while a often shows movement toward a place.

• en el restaurante = in / at the restaurant
• al restaurante = to the restaurant

So:

• En el restaurante pedimos... = while we were there, in that setting
• Fuimos al restaurante = we went to the restaurant

They express different ideas.

In Spanish, only two preposition + article contractions are standard:

• a + el = al
• de + el = del

But:

• en + el stays en el

So en el restaurante is exactly what you should expect.

After verbs like pedir, Spanish often leaves out the article when talking about some amount of food rather than a specific, already-identified item.

So pedimos mejillones sounds natural for we ordered mussels.

Compare:

• Pedimos mejillones = we ordered mussels
• Pedimos los mejillones = we ordered the mussels, meaning specific mussels already known in context

This is very common with food and drink:

• Pedimos pan
• Tomé café
• Compraron vino

Para + infinitive is a very common structure in Spanish. It often means:

• in order to ...
• for ...-ing
• to ...

So para compartir literally works like to share or for sharing.

It explains the purpose of ordering the mussels: they were ordered to share.

Other examples:

• agua para beber = water to drink
• una mesa para trabajar = a table to work at
• algo para comer = something to eat

Because after para, Spanish normally uses the infinitive when the subject is general or obvious.

• para compartir = to share / for sharing

If you said para compartimos, that would be ungrammatical.

A full clause with a conjugated verb would need a different structure, for example:

• para que compartamos = so that we share

But in your sentence, that would sound heavier and much less natural than para compartir.

Yes, very common. In Spain, para compartir is often used in restaurants and on menus to mean that a dish is meant to be shared among several people.

You might see things like:

• raciones para compartir
• platos para compartir

This fits Spanish dining culture well, especially with tapas, starters, or seafood dishes that people eat together.

In Spanish, pedir is the normal verb for asking for or ordering food and drinks in a restaurant.

So in Spain, it is very natural to say:

• Pedimos mejillones
• Voy a pedir una tortilla
• ¿Qué vas a pedir?

A direct equivalent of English to order, ordenar, exists, but in Spain it is usually less natural in everyday restaurant language. Pedir is the verb learners should normally use.

El restaurante can be used because the restaurant is treated as a known setting or a specific place already understood in context.

Spanish often uses the definite article in situations where English might also use the, especially when the place is already part of the situation being described.

• En el restaurante... sounds like at the restaurant / in the restaurant
• En un restaurante... would mean in a restaurant, introducing it as nonspecific

So the choice depends on whether the speaker has a particular restaurant or established setting in mind.