Era para jantarmos marisco, mas no fim fizemos uma omelete com espinafres.

Here era para expresses an expectation, plan, or intention. A natural English equivalent is was supposed to or the plan was to.

So Era para jantarmos marisco suggests that seafood was the original plan, but that plan did not happen.

Jantarmos is the personal infinitive. Portuguese can use an infinitive that changes depending on the subject:

• jantar = to have dinner
• jantarmos = for us to have dinner

After para, Portuguese often uses the infinitive, and when the subject is clear and specific, the personal infinitive is very common. So:

• para jantar = to have dinner / for someone to have dinner
• para jantarmos = for us to have dinner

In this sentence, jantarmos makes the subject we explicit.

Era is the imperfect of ser, and it works well here because it describes a previous plan or expectation in the background.

Era para... mas... is a very common pattern for saying:

• It was supposed to... but...
• The idea was to... but...

Using foi would sound less natural in this structure. Era para is the usual choice when talking about something that was intended but did not happen.

Portuguese often leaves out subject pronouns when the verb form already shows who the subject is.

Here, both verbs make the subject clear:

• jantarmos = we
• fizemos = we

So nós is unnecessary. You could add it for emphasis, but normally Portuguese prefers to omit it.

Here it is a verb.

Portuguese jantar can be:

• a noun: o jantar = dinner
• a verb: jantar = to have dinner / to eat for dinner

In Era para jantarmos marisco, it is clearly the verb form.

In European Portuguese, marisco usually refers to shellfish and related seafood such as prawns, crab, lobster, clams, mussels, and so on.

Depending on context, English learners may see it translated as seafood, but shellfish is often closer. It does not usually mean ordinary fish in the broad sense of peixe.

This is because the two nouns are being used differently.

• marisco is being used in a general food sense: have seafood for dinner
• uma omelete refers to one specific dish that they made

So:

• jantarmos marisco = eat seafood / have seafood
• fizemos uma omelete = we made an omelette

In Portuguese, food items used generically often appear without an article.

No fim literally means in the end or at the end, but in this sentence it means something like:

• in the end
• eventually
• as it turned out

It marks the final outcome, especially when it contrasts with an earlier plan.

Using fazer with dishes is very common in Portuguese.

So fizemos uma omelete is a normal way to say:

• we made an omelette

Portuguese often uses fazer where English uses make for meals and dishes:

• fazer sopa
• fazer um bolo
• fazer uma omelete

You could say cozinhámos, but that is broader and less specific. Fizemos uma omelete focuses on the dish they prepared.

That is just a difference between the two languages. In Portuguese, espinafres is normally plural when talking about spinach as a food.

So:

• English: spinach
• Portuguese: espinafres

This is one of those vocabulary differences that does not map neatly word for word.

Most naturally, uma omelete com espinafres means an omelette with spinach in it or made with spinach.

Grammatically, com just means with, so context always matters, but with a dish like omelete, the usual interpretation is that spinach is one of the ingredients.

Yes. In European Portuguese, omelete is the standard form.

English speakers may also come across omeleta, which is more associated with Brazilian Portuguese. Since this sentence is European Portuguese, omelete is exactly what you would expect.

Yes, you could, but the meaning shifts slightly.

• Era para jantarmos marisco = the plan was for seafood to be the dinner
• Era para comermos marisco = the plan was for us to eat seafood

Both are correct, but jantarmos fits especially well because the sentence is about what they were going to have for dinner. It sounds a bit more natural in this context.