Hún setur tannþráðinn við hliðina á sjampóinu svo hún gleymi honum ekki aftur.

Because Icelandic usually puts the definite article, the, at the end of the noun.

• tannþráður = dental floss / floss
• tannþráðinn = the dental floss

It is also the direct object of setur (puts), so it appears in the accusative singular definite form.

So:

• dictionary form: tannþráður
• in this sentence: tannþráðinn

It is a compound, and Icelandic makes compounds very often.

• tann- = tooth / dental
• þráður = thread

So tannþráður is literally tooth-thread, meaning dental floss.

This is very typical in Icelandic. Compare:

• tannbursti = toothbrush
• tannkrem = toothpaste

A useful habit in Icelandic is to look for smaller words inside longer ones.

Because the sentence means the shampoo, and this form is required by the expression við hliðina á.

Here is the breakdown:

• sjampó = shampoo
• sjampóið = the shampoo (basic definite form)
• sjampóinu = the shampoo in the dative singular

So sjampóinu is there because the phrase við hliðina á takes a dative noun after á.

Við hliðina á is a fixed expression meaning next to or beside.

It is best learned as one chunk:

• við hliðina á + dative = next to / beside

So:

• við hliðina á sjampóinu = next to the shampoo

Even though it looks word-for-word like more than one little piece, learners usually do best if they memorize the whole expression together instead of trying to translate each part separately every time.

Because svo here means so that, and the clause expresses purpose. In Icelandic, purpose clauses like this often use the subjunctive.

So:

• gleymir = ordinary present indicative, forgets
• gleymi = present subjunctive

In this sentence:

• svo hún gleymi honum ekki aftur = so that she does not forget it again

You may also see the fuller form:

• svo að hún gleymi honum ekki aftur

Because honum refers back to tannþráðinn, and the verb gleyma requires the dative.

So two things are happening at once:

• the pronoun refers to tannþráðinn, which is masculine singular
• gleyma takes a dative object

That gives:

• honum = him/it in the dative masculine singular

So gleymi honum means forget it, with the correct Icelandic case.

Repeating the noun would also be grammatically possible in some contexts, but using the pronoun is more natural once the noun has already been mentioned.

Because short object pronouns in Icelandic often come before ekki.

So:

• gleymi honum ekki = do not forget it

This word order is very common and natural. Compare:

• Ég sé hann ekki = I do not see him
• Hún þekkir hana ekki = She does not know her

For English speakers, this can feel unusual at first, because English keeps not earlier: do not forget it. Icelandic often places the pronoun before ekki.

Here aftur means again.

So:

• ekki aftur = not again

That gives the sense:

• so that she does not forget it again

Be aware that aftur can also mean back in other contexts, so the exact meaning depends on the sentence.

The first clause has normal Icelandic main-clause word order:

• Hún setur tannþráðinn við hliðina á sjampóinu
• She puts the floss next to the shampoo

Here the subject comes first and the finite verb comes second:

• Hún = subject
• setur = finite verb

That fits Icelandic’s usual V2 pattern in main clauses.

The second clause is a subordinate clause:

• svo hún gleymi honum ekki aftur

In subordinate clauses, the word order is different from main-clause V2, so hún stays before gleymi.

A few helpful pronunciation points:

• þ is like th in thing
• ð is like th in this, though often softer in real speech
• sj is usually like sh

So in this sentence:

• þ in tannþráðinn has the thing sound
• ð in þráð- is a soft voiced th
• sj in sjampóinu sounds like sh

Also, Icelandic words are usually stressed on the first syllable, which helps a lot when reading longer words.