Ef við gleymum að vökva, byrja túlípanarnir að visna fljótt.

Ef means if and introduces a condition. The first part, Ef við gleymum að vökva, is the if-clause: it sets up the situation under which the second part happens.

So the structure is:

Ef ... , ... = If ... , ...

Gleymum is the 1st person plural present tense of gleyma — to forget.

So:

• ég gleymi = I forget
• við gleymum = we forget

Here, við gleymum means we forget.

Yes. Gleyma often takes the dative when it has a noun object.

For example:

• Ég gleymdi lyklunum. = I forgot the keys.

Here, though, gleyma is followed by an infinitive phrase:

• að vökva = to water

So instead of forgetting a noun, the sentence means forgeting to do something. That is why you do not see a separate dative noun after gleymum.

In this sentence, að is the infinitive marker, like English to in to water.

So:

• að vökva = to water

Many Icelandic verbs are followed by að + infinitive, and gleyma is one of them when it means forget to do something.

The object is simply understood from context. Icelandic often leaves out an object when it is obvious.

Here, the tulips are mentioned in the main clause, so að vökva is understood as to water them / to water the tulips.

A fuller version could say something like að vökva túlípanana, but that is not necessary if the meaning is already clear.

This is a very common Icelandic word-order pattern.

The sentence begins with a subordinate clause:

Ef við gleymum að vökva, ...

After that, the main clause follows the verb-second pattern. That means the finite verb comes early in the clause, so you get:

• byrja túlípanarnir ...

rather than

• túlípanarnir byrja ...

This is similar to inversion after a fronted element in some other Germanic languages.

So:

• Túlípanarnir byrja að visna fljótt. = The tulips begin to wilt quickly.
• Ef við gleymum að vökva, byrja túlípanarnir að visna fljótt. = If we forget to water, the tulips begin to wilt quickly.

Túlípanarnir means the tulips.

It is the definite plural nominative form of túlípani.

Breakdown:

• túlípani = a tulip
• túlípanar = tulips
• túlípanarnir = the tulips

The ending -nir / -arnir is a typical way Icelandic marks the definite plural on masculine nouns.

Because byrja is commonly followed by að + infinitive when it means begin to do something.

So:

• byrja að visna = begin to wilt

This is very similar to English begin to wilt.

Visna means to wilt, to wither, or to fade from lack of water or freshness.

It is an intransitive verb here, which means the tulips themselves are undergoing the change:

• Túlípanarnir visna. = The tulips wilt.

So the sentence does not mean that someone is wilting them; it means they start wilting on their own because they were not watered.

Fljótt means quickly. It is an adverb modifying að visna.

So:

• að visna fljótt = to wilt quickly

It tells you how fast the wilting starts to happen.

Yes. Icelandic often allows þá in conditional sentences, especially for emphasis:

• Ef við gleymum að vökva, þá byrja túlípanarnir að visna fljótt.

That would mean the same thing: If we forget to water, then the tulips begin to wilt quickly.

Without þá, the sentence is still completely natural.