Ég verð stundum sjóveikur þegar bylgjurnar eru stórar, en systir mín getur alltaf siglt.

Verð is from verða, which often means become or get. So ég verð sjóveikur means I get seasick.

If you used er instead, ég er sjóveikur would mean I am seasick. That describes the state itself, while verða emphasizes entering that state.

Because the adjective has to agree with the speaker in gender, number, and case.

Here:

• ég = I
• the subject is singular nominative
• sjóveikur is the masculine nominative singular form

So this sentence sounds like it is spoken by a male speaker.

If a female speaker said it, she would normally say:

• Ég verð stundum sjóveik þegar bylgjurnar eru stórar.

It is an adjective meaning seasick.

It is a compound:

• sjór = sea
• veikur = sick, weak

Icelandic often builds words this way. So sjóveikur is literally something like sea-sick.

Stundum means sometimes.

Its position here is very natural:

• Ég verð stundum sjóveikur ...

In Icelandic, adverbs like stundum often come after the finite verb in a main clause.

You can also move it for emphasis:

• Stundum verð ég sjóveikur ...

That still works, but the sentence then starts with sometimes for focus.

Here it means when, and in context it can also feel like whenever.

So the idea is not a single event only, but a repeated situation:

• whenever the waves are big, I sometimes get seasick

If you wanted a more purely conditional if, Icelandic would usually use ef instead.

Because bylgjurnar is the definite form: the waves.

Breakdown:

• bylgjur = waves
• bylgjurnar = the waves

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

So:

• bylgjur = waves
• bylgjurnar = the waves

Because stórar is a predicate adjective, not an adjective placed directly before the noun.

In bylgjurnar eru stórar:

• bylgjurnar = the waves
• eru stórar = are big

The adjective agrees with bylgjurnar:

• bylgja is feminine
• plural here = feminine plural
• so the adjective is stórar

A useful comparison:

• Bylgjurnar eru stórar = the waves are big
• Stóru bylgjurnar = the big waves

In the second example, the adjective is directly attached to the definite noun phrase, so the form changes.

In ordinary Icelandic, possessives often come after the noun:

• systir mín = my sister

That is the most neutral, everyday way to say it.

Mín systir is possible, but it usually sounds more emphatic, contrastive, literary, or marked in some way.

So for a plain statement, systir mín is the normal choice.

Because geta commonly takes the sagnbót form of the following verb.

For sigla:

• infinitive: sigla
• sagnbót: siglt

So:

• getur siglt = can sail

This is a pattern learners meet often after geta:

• getur lesið
• getur farið
• getur séð

So siglt is the expected form here.

That is a very common Icelandic word order.

Here:

• getur is the finite verb
• alltaf is an adverb
• siglt is the non-finite verb form

So the pattern is:

• finite verb + adverb + non-finite verb

That is why you get:

• systir mín getur alltaf siglt

This placement sounds natural and idiomatic in Icelandic.