Ég set tannþráðinn við hliðina á tannkreminu.

It is present tense.

The verb is setja (to put / to place / to set), and set is its 1st person singular present form, so with ég it means I put or I am putting, depending on context.

A useful comparison:

• ég set = I put / I am putting
• ég setti = I put / I placed in the past

So even though set looks like English set, in Icelandic here it is definitely present, not past.

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

So here:

• tannþráðinn = the dental floss
• hliðina = the side
• tannkreminu = the toothpaste

This is called the suffixed definite article.

So instead of a separate word like English the, Icelandic often builds definiteness directly into the noun.

Because tannþráðinn is the accusative singular definite form, and the noun is the direct object of set.

The basic dictionary form is:

• tannþráður = dental floss / literally tooth-thread

But as a direct object after setja, it goes into the accusative:

• nominative: tannþráður
• accusative: tannþráð
• accusative definite: tannþráðinn

So the -ur you see in the dictionary form is a nominative ending. It disappears in the accusative.

Because in the phrase við hliðina á ..., the noun after á is in the dative.

The base noun is:

• tannkrem = toothpaste

Its definite forms include:

• nominative definite: tannkremið = the toothpaste
• dative definite: tannkreminu

So á tannkreminu means at/on the toothpaste in form, but inside this full expression it corresponds to English next to the toothpaste.

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

Literally, it is something like:

• við = by / at
• hliðina = the side
• á = of / at / on, depending on context

So the whole phrase is roughly by the side of.

English uses a single phrase next to, and Icelandic uses við hliðina á. It is best to learn it as one unit rather than trying to translate each word separately every time.

Because við takes the accusative, and hliðina is the accusative definite form in this expression.

So in:

• við hliðina á tannkreminu

the word hliðina is the object of við, while tannkreminu belongs to the following á phrase.

A good way to think of it is:

• við
  • hliðina
• á
  • tannkreminu

This split helps explain why the two nouns have different endings.

Yes.

Icelandic often uses the simple present where English might use either:

• I put
• I am putting

So Ég set tannþráðinn við hliðina á tannkreminu can describe:

• a habitual action, depending on context, or
• something happening right now

Icelandic does not use a separate progressive form as much as English does.

Yes, both are compounds, and that is very common in Icelandic.

They break down like this:

• tann- = tooth
• þráður = thread
• tannþráður = dental floss (literally tooth-thread)

and

• tann- = tooth
• krem = cream
• tannkrem = toothpaste (literally tooth-cream)

A useful rule: in Icelandic compounds, the last part usually determines the word’s gender and how it declines.

These two letters are very important in Icelandic:

• þ is like th in thin
• ð is like th in this

So:

• þráðinn starts with the thin sound
• hliðina contains the this sound

A few rough guides:

• tannþráðinn ≈ tahnn-thrau-thin
• hliðina ≈ hlith-ih-na

The exact Icelandic pronunciation is a bit more precise than English spelling can show, but those comparisons are a good start.

No. In Icelandic, accent marks are part of the spelling and usually show a different vowel sound, not just stress.

For example:

• é
• á
• í

are treated as distinct vowel letters, not just ordinary vowels with stress marks.

Also, Icelandic stress normally falls on the first syllable of the word anyway, so the accents are not mainly there to mark stress.

Yes. This is a very normal, neutral sentence pattern:

• Ég = subject
• set = verb
• tannþráðinn = direct object
• við hliðina á tannkreminu = place expression

So the structure is basically:

Subject + Verb + Object + Location

Icelandic can change word order for emphasis or style, but this version is straightforward and natural.