Hún segir að blái kjóllinn sé fallegri, en ég veit samt ekki hvaða kjól ég á að velja.

Sé is the present subjunctive form of vera (to be).

In a sentence like Hún segir að ..., Icelandic often uses the subjunctive when reporting someone else’s statement, opinion, or judgment rather than presenting it as a plain fact from the speaker’s own point of view. Here, sé fallegri sounds like reported opinion: she says it is prettier.

So:

• er = ordinary present is
• sé = subjunctive be/is in a subordinate clause

A learner should mainly recognize sé here as a form of vera.

Because blái kjóllinn means the blue dress, while blár kjóll means a blue dress.

In Icelandic, when an adjective modifies a definite noun, the adjective usually takes the weak form, and the noun carries the suffixed definite article.

So:

• blár kjóll = a blue dress
• blái kjóllinn = the blue dress

Here:

• blái = weak form of blár (blue)
• kjóllinn = the dress

That is normal Icelandic grammar.

In a definite noun phrase with an adjective, Icelandic typically shows definiteness in two ways:

1. the adjective appears in its weak form
2. the noun takes the suffixed definite article

So blái kjóllinn is the normal way to say the blue dress.

This is different from English, where only the shows definiteness.

Fallegri is the comparative form of fallegur (beautiful, pretty).

So:

• fallegur = beautiful / pretty
• fallegri = more beautiful / prettier

In this sentence, sé fallegri means is prettier.

The comparison is understood from context.

Fallegri is a comparative, so it implies comparison with another dress or dresses already being discussed. Icelandic, like English, can leave the second part of the comparison unspoken when it is obvious.

So this can mean something like:

• the blue dress is prettier
  = prettier than the other one / prettier than the others

If the speaker wanted to state it explicitly, they could add a phrase with en meaning than.

Because kjóll is the object of velja (to choose), and velja takes the accusative case.

So:

• kjóll = nominative form
• kjól = accusative form

Since the sentence means which dress I should choose, dress is the thing being chosen, so it appears as kjól.

No, hvaða itself stays the same here. The noun after it shows the case.

So the important change is:

• kjóll → kjól

This is why you get hvaða kjól = which dress in this sentence.

Yes. Á is the present tense of eiga, but in the expression eiga að + infinitive, it does not mean possession.

Instead, eiga að means something like:

• should
• am supposed to
• have to

So:

• ég á að velja = I should choose / I am supposed to choose

This is a very common Icelandic construction.

Because this is an embedded question, not a direct question.

Direct question:

• Hvaða kjól á ég að velja?
  = Which dress should I choose?

Embedded question:

• ... ég veit ekki hvaða kjól ég á að velja.
  = ... I don’t know which dress I should choose.

In embedded clauses, Icelandic does not use the same question word order as in direct questions. This is similar to English:

• direct: Which dress should I choose?
• embedded: I don’t know which dress I should choose.

They have different jobs.

The first að is a conjunction meaning that:

• Hún segir að ... = She says that ...

The second að is the infinitive marker to:

• ég á að velja = I am supposed to choose

So even though both words look the same, they are not doing the same thing.

Samt means still, nevertheless, or all the same.

In this sentence, it adds contrast:

• Hún segir að blái kjóllinn sé fallegri
• en ég veit samt ekki ...

So the sense is:

• She says the blue dress is prettier, but I still don’t know which dress to choose.

It shows that her opinion has not completely solved the speaker’s problem.