Ég pantaði mildan forrétt, en systir mín valdi kryddaðan aðalrétt.

En means but here. It links the two clauses and shows a contrast:

• Ég pantaði mildan forrétt = I ordered a mild starter
• en systir mín valdi kryddaðan aðalrétt = but my sister chose a spicy main course

In some contexts, en can also feel like while, but but is the most natural translation in this sentence.

Icelandic does not have an indefinite article like English a/an. So a bare noun can mean a or an depending on context.

That means:

• mildan forrétt = a mild appetizer
• kryddaðan aðalrétt = a spicy main course

If the noun were definite, Icelandic usually adds the article to the end of the noun:

• forrétturinn = the appetizer
• aðalrétturinn = the main course

Because they are adjectives agreeing with masculine singular nouns in the accusative case.

Here the direct objects are:

• forrétt from forréttur
• aðalrétt from aðalréttur

Both are masculine singular, and after these verbs they are in the accusative. The adjectives must match that, so:

• mildur → mildan
• kryddaður → kryddaðan

This is a very common pattern in Icelandic.

The dictionary form of these nouns is nominative singular:

• forréttur
• aðalréttur

But in this sentence they are direct objects, so they appear in the accusative singular. For many masculine nouns ending in -ur, that -ur disappears in the accusative:

• forréttur → forrétt
• aðalréttur → aðalrétt

So the noun form changes because of case.

They are both in the past tense.

• pantaði comes from að panta = to order
• valdi comes from að velja = to choose

So:

• Ég pantaði = I ordered
• systir mín valdi = my sister chose

Notice that Icelandic past-tense verb forms do not change for person in the same way English does here. Pantaði and valdi can be used with both I and he/she depending on context.

In Icelandic, possessives often come after the noun, so systir mín is the normal way to say my sister.

• systir mín = my sister

You can sometimes put the possessive first, but that usually sounds more emphatic, literary, or contrastive. So for everyday neutral speech, systir mín is the form learners should expect most often.

Yes, it follows a very typical Icelandic main-clause pattern.

The first clause is:

• Ég
  • pantaði
    • mildan forrétt

The second clause is:

• systir mín
  • valdi
    • kryddaðan aðalrétt

This is a normal subject–verb–object order. More broadly, Icelandic is a verb-second language in main clauses, which means the finite verb tends to come early in the clause. In this sentence, the subject comes first, so the verb naturally comes second.

They are compound words built around réttur, which in this context means dish or course.

• forréttur = starter / appetizer
  • for- suggests something that comes before
• aðalréttur = main course
  • aðal- means main or principal

So they are very transparent once you know the parts.

Not always. Kryddaður literally means spiced or seasoned. Depending on context, it can mean:

• spicy
• well-seasoned
• flavored with spices

In this sentence, spicy is the most natural translation, especially because it contrasts with mildan = mild.

You would change the verbs to present tense:

• Ég panta mildan forrétt, en systir mín velur kryddaðan aðalrétt.

That means:

• I order a mild appetizer, but my sister chooses a spicy main course.

So the main changes are:

• pantaði → panta
• valdi → velur