Hann ætlaði ekki að panta forrétt, en endaði á að fá milda súpu sem aðalrétt.

Ætla að + infinitive is a very common Icelandic pattern meaning to intend to, to plan to, or often to be going to.

So:

• hann ætlar að panta = he intends/is going to order
• hann ætlaði að panta = he intended/was going to order

In your sentence, ætlaði is the past tense, 3rd person singular, because the whole sentence is talking about what he intended at that time.

That is normal Icelandic word order.

In a main clause, the finite verb usually comes early in the sentence, and ekki often comes right after it. So:

• Hann ætlaði ekki að panta forrétt.

This is the natural order. A native English speaker may want to copy English not placement, but Icelandic does not do that here.

A useful pattern is:

• Subject + finite verb + ekki + rest

For example:

• Ég kann ekki að synda. = I can’t swim.
• Hún vildi ekki fara. = She didn’t want to go.

They belong to two different structures.

1. ætla að + infinitive

   • ætlaði að panta
   • intended to order

2. enda á að + infinitive

   • endaði á að fá
   • ended up getting

So both að words are introducing an infinitive verb, but they are part of different expressions.

Enda á að + infinitive is an idiomatic expression meaning to end up doing something.

So:

• endaði á að fá = ended up getting

It does not mean simply finished in a literal sense here. It means that the final result was different from what was expected or intended.

Examples:

• Hún endaði á að kaupa bókina. = She ended up buying the book.
• Við enduðum á að vera heima. = We ended up staying home.

Because forréttur is the nominative singular dictionary form, but here the word is the object of panta, so it is in the accusative singular.

For many masculine nouns ending in -ur, that -ur disappears in the accusative singular.

So:

• forréttur = nominative
• forrétt = accusative

The same thing happens with many other masculine nouns.

For the same reason as forrétt: the dictionary form is aðalréttur, but here Icelandic uses aðalrétt.

In sem aðalrétt, the noun means as a main course, and it appears in the form used in this construction, which is aðalrétt.

So the important thing to recognize is:

• aðalréttur = dictionary / nominative form
• aðalrétt = the form actually used here

Because milda súpu is the object of fá, so it must be in the accusative, and the adjective has to agree with the noun.

The noun:

• súpa = nominative
• súpu = accusative

The adjective:

• mild = nominative feminine singular
• milda = accusative feminine singular

So:

• mild súpa = a mild soup (subject form)
• milda súpu = a mild soup (object form)

This agreement between adjective and noun is very important in Icelandic.

Here, sem means as, not that/which.

So:

• milda súpu sem aðalrétt = a mild soup as a main course

This is a common use of sem to show a role or function.

Compare:

• Hann vann sem kennari. = He worked as a teacher.
• Ég fékk fisk sem forrétt. = I got fish as a starter.

So yes, sem can have different uses, and in this sentence it means as.

Yes, fá literally means get, but in Icelandic it is very common when talking about food, meals, or what someone is served.

So in a restaurant context:

• panta focuses on the act of ordering
• fá focuses on what someone gets / is served / ends up having

That is why the sentence says:

• he didn’t intend to order a starter
• but he ended up getting a mild soup as a main course

This sounds natural in Icelandic.

Because Icelandic does not have a separate indefinite article like English a/an.

So a bare noun can mean:

• forrétt = a starter
• súpu = a soup
• aðalrétt = a main course

If Icelandic wants to make the noun definite, it usually adds the definite article to the end of the word:

• forrétturinn = the starter
• súpan = the soup
• aðalrétturinn = the main course

So the lack of a separate word for a is completely normal.

En means but and connects the two clauses:

• Hann ætlaði ekki að panta forrétt
• en endaði á að fá milda súpu sem aðalrétt

It introduces a contrast:

• what he intended vs.
• what actually happened

So it works very much like English but.