Jógúrt með bláberjum er gott snarl þegar ég hef litla matarlyst á morgnana.

Because gott describes snarl, not jógúrt.

The structure is:

• Jógúrt með bláberjum = the subject
• er = is
• gott snarl = a good snack

So the sentence means Yogurt with blueberries is a good snack.
Here, snarl is a neuter singular noun, so the adjective must also be neuter singular: gott.

If the adjective were describing jógúrt, then you would expect a feminine form instead.

Because it comes after með, and here með takes the dative.

• bláber = blueberries
• bláberjum = dative plural

So:

• með bláberjum = with blueberries

A very useful rule is:

• með + dative when it means with
• með + accusative in some other meanings, such as motion or opposition

In this sentence, it clearly means accompaniment, so dative is used.

It is accusative singular.

That is because hafa normally takes a direct object in the accusative:

• ég hef = I have
• litla matarlyst = little appetite

More specifically:

• matarlyst is a feminine singular noun
• the accusative singular form of lítill here is litla

So the adjective and noun match in gender, number, and case.

Á morgnana means in the mornings or in the morning(s) habitually.

This is a common Icelandic time expression:

• á morgnana = in the mornings
• á kvöldin = in the evenings
• um helgarnar = on weekends

So this sentence is talking about a repeated situation, not just one morning.

Also, this is different from:

• á morgun = tomorrow

Those can look similar to learners, but they mean different things.

Because Icelandic has no indefinite article.

So English a good snack is simply:

• gott snarl

Icelandic can express definiteness, but usually with:

• a suffixed definite article on the noun, or
• context

So bare nouns often correspond to English a/an.

It is a compound noun meaning appetite.

It is made from:

• matar- = food-related / of food
• lyst = desire, liking, appetite

So literally it is something like desire for food.

This is very typical Icelandic: compound nouns are everywhere, and learning to spot the parts can help a lot.

Because þegar introduces a subordinate clause.

In that clause, the normal order is:

• þegar ég hef litla matarlyst
• when I have little appetite

So after þegar, you usually get regular subject-verb order:

• ég hef

This is different from some main-clause patterns in Icelandic, where the finite verb often appears in second position.

Yes. You could also say:

• Þegar ég hef litla matarlyst á morgnana, er jógúrt með bláberjum gott snarl.

That means the same thing.

When the subordinate clause comes first, the main clause still follows Icelandic word-order rules, so the finite verb er comes before the subject jógúrt með bláberjum.

That kind of word order shift is very common in Icelandic.

Because the subject is singular:

• Jógúrt = yogurt

The phrase með bláberjum is just an added prepositional phrase modifying jógúrt. It does not make the subject plural.

So the whole subject is still grammatically singular, and the verb is therefore:

• er = is

It sounds like a general habit or a usual situation.

That comes from two things:

• hef is in the present tense
• á morgnana means in the mornings, repeatedly

So the sense is:

• Yogurt with blueberries is a good snack when I have little appetite in the mornings

If the speaker meant just one specific morning, Icelandic would usually use a different time expression.