Ég nota ekki sláttuvélina þegar börnin leika sér nálægt runnanum.

In a normal Icelandic main clause, the finite verb usually comes early in the sentence, and ekki normally comes after that verb.

So:

• Ég nota ekki sláttuvélina = correct, natural order
• Ég ekki nota sláttuvélina = not standard Icelandic word order

This is one of the first big word-order differences English speakers notice.

Nota is the 1st person singular present tense of að nota, meaning to use.

So:

• ég nota = I use / I am using
• að nota = to use

In this sentence, the present tense gives a general or habitual meaning: the speaker is saying this is something they do not do when the children are nearby.

Because it is:

• the direct object of nota
• singular
• definite = the lawnmower
• a feminine noun
• in the accusative case

The base noun is sláttuvél = lawnmower.

Here is the rough pattern:

• sláttuvél = a lawnmower
• sláttuvélin = the lawnmower (nominative)
• sláttuvélina = the lawnmower (accusative)

Since nota takes a direct object, Icelandic uses the accusative, so you get sláttuvélina.

In Icelandic, the definite article is usually attached to the noun instead of being a separate word.

So instead of having a separate word like English the, Icelandic often adds an ending:

• sláttuvél = lawnmower
• sláttuvélin / sláttuvélina = the lawnmower

The same thing happens with:

• börn = children
• börnin = the children

and

• runni = bush / shrub
• runnanum = the bush / shrub

Because börnin means the children, while börn would mean just children.

Also, börnin is the subject of the clause þegar börnin leika sér ..., so it is in the nominative.

The noun barn is a neuter noun:

• barn = child
• börn = children
• börnin = the children

So börnin leika sér means the children are playing.

Leika sér is a very common Icelandic expression meaning to play in the sense of children playing, playing around, amusing themselves, and so on.

So:

• börnin leika sér = the children are playing

If you remove sér, the meaning changes. Leika by itself often means things like:

• perform / act
• play a role
• play something

So for ordinary children playing, Icelandic normally uses leika sér.

Because this expression requires the dative reflexive pronoun, not the accusative one.

The reflexive forms are:

• sig = accusative
• sér = dative
• sín = genitive

The idiom is leika sér, so the correct form is sér.

This is not something you can predict from English very easily; it is mostly something you learn as part of the whole expression.

Nálægt means near / close to, and it normally takes the dative case.

The noun runni means bush / shrub. In this sentence it is:

• singular
• definite
• dative

So:

• runni = bush / shrub
• runninn = the bush (nominative)
• runnanum = the bush (dative)

That is why the sentence has nálægt runnanum = near the bush.

Here þegar means when.

It introduces a time clause:

• þegar börnin leika sér nálægt runnanum = when the children are playing near the bush

In context, the whole sentence has a general/habitual sense, so English might also translate it naturally as something like when(ever) the children play near the bush.

Because the sentence describes a general situation or habit, not just one single event happening right now.

So Icelandic uses the present tense for both parts:

• Ég nota ekki ... = I do not use ...
• þegar börnin leika sér ... = when the children play / are playing ...

English does something similar in general statements:

• I don’t use the lawnmower when the children are playing nearby.

Yes. If you put the time clause first, the main clause still keeps normal Icelandic verb-second behavior.

So you can say:

• Þegar börnin leika sér nálægt runnanum, nota ég ekki sláttuvélina.

Notice that after the opening clause, the finite verb nota comes before ég:

• ... nota ég ...
• not ... ég nota ...

This is a very important Icelandic word-order pattern.