Miðinn var gildur í gær.

Miði is the basic form, meaning ticket.
Miðinn means the ticket.

Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the.

• miði = a ticket
• miðinn = the ticket

So Miðinn var gildur í gær starts with the ticket, not just ticket.

Miðinn is in the nominative singular.

That is because it is the subject of the sentence: it is the thing being described. The sentence is saying something about the ticket, namely that it was valid yesterday.

With vera (to be), the subject normally stays in the nominative:

• Miðinn var gildur = The ticket was valid

Var is the past tense of vera, which means to be.

In this sentence, it means was.

A few useful forms are:

• ég var = I was
• þú varst = you were
• hann / hún / það var = he / she / it was

Since miðinn is a third-person singular subject, var is the correct form.

Because Icelandic adjectives agree with the noun they describe in gender, number, and case.

Here, miðinn is:

• masculine
• singular
• nominative

So the adjective also appears in the masculine singular nominative form:

• gildur = masculine
• gild = feminine
• gilt = neuter

That is why the sentence has gildur.

Because this is a predicate adjective: it comes after vera and describes the subject.

In Icelandic, predicate adjectives normally use the strong form, even when the noun is definite.

Compare:

• góði miðinn = the good ticket
  
  • here the adjective comes before the noun, so it is an attributive adjective
• miðinn er góður = the ticket is good
  
  • here the adjective comes after er, so it is a predicate adjective

The same thing happens here:

• miðinn var gildur = the ticket was valid

Because í gær is the normal Icelandic expression for yesterday.

It is best learned as a fixed time phrase rather than translated word-for-word. Even though í often means in, the full expression í gær simply means yesterday.

Similar time expressions are common in Icelandic, so this is a good phrase to memorize as a unit.

Yes. That is a completely natural sentence.

Icelandic main clauses usually follow the V2 pattern, which means the finite verb stays in the second position.

So both are correct:

• Miðinn var gildur í gær.
• Í gær var miðinn gildur.

The second version puts more emphasis on yesterday.

Yes.

That uses the verb gilda, which means to be valid or to be in force.

So you have two natural ways to express the idea:

• Miðinn var gildur í gær.
  
  • literally The ticket was valid yesterday
• Miðinn gilti í gær.
  
  • literally The ticket was valid / held good yesterday

The first uses vera + adjective.
The second uses a single verb.

The dictionary forms are:

• miði = ticket
• vera = to be
• gildur = valid
• gær appears mainly in the fixed expression í gær = yesterday

This is useful because Icelandic words often appear in changed forms in real sentences, so learners need to know what to look up.

It would be:

Miðarnir voru gildir í gær.
The tickets were valid yesterday.

Notice how several words change:

• miðinn → miðarnir
• var → voru
• gildur → gildir

This shows an important Icelandic pattern: nouns, verbs, and adjectives often all change to match the grammar of the sentence.