Tannlæknirinn sagði að ég hefði átt að nota tannþráð oftar áður en mér fór að verða illt í tönninni.

Tannlæknir means dentist.
Tannlæknirinn means the dentist.

Icelandic usually adds the definite article to the end of the noun instead of using a separate word like the. So:

• tannlæknir = a dentist / dentist
• tannlæknirinn = the dentist

In this sentence, Tannlæknirinn sagði... means The dentist said...

This whole part means something like said that I should have used.

Breakdown:

• sagði = said
• að = that
• ég hefði átt að = I should have / I was supposed to
• nota = use

So:

• ég hefði átt að nota tannþráð = I should have used dental floss

A very useful pattern is:

• hafa átt að + infinitive

In past contexts, this often corresponds to English should have done.

This is a very common learner question.

Átti að can mean was supposed to or should in a more direct way.
But hefði átt að is often used for should have in the sense of regret, advice, or something that would have been better to do earlier.

Here the dentist is looking back and saying what would have been advisable, so hefði átt að nota fits well.

There is also another point: after a past reporting verb like sagði, Icelandic often uses the subjunctive in indirect speech. Hefði is the past subjunctive form of hafa.

So ég hefði átt að nota tannþráð is a very natural way to say:

• I should have used dental floss
• I ought to have used dental floss

Because tannþráð is the accusative form, and nota normally takes a direct object in the accusative.

The noun is:

• nominative: tannþráður
• accusative: tannþráð

So:

• tannþráður = dental floss
• nota tannþráð = use dental floss

This is very normal Icelandic case grammar: the verb nota takes the accusative object.

Oftar means more often.

It is the comparative form of oft:

• oft = often
• oftar = more often

So:

• nota tannþráð oftar = use dental floss more often

Notice that Icelandic does not need an extra word like more here, because the comparative ending already carries that meaning.

Áður en means before when it introduces a full clause.

So here:

• áður en mér fór að verða illt í tönninni
  = before my tooth started to hurt

A useful distinction:

• áður = earlier / before
• áður en = before + clause

Examples:

• Ég kom áður. = I came earlier.
• Ég kom áður en hann fór. = I came before he left.

This is a very idiomatic Icelandic part.

• fara að + infinitive = begin to / start to
• verða illt = become painful / start hurting
• mér = to me / for me

So:

• mér fór að verða illt literally means something like it began to become painful for me
• naturally, it means it started to hurt

In the full sentence:

• áður en mér fór að verða illt í tönninni
  = before my tooth started to hurt

Even though fór is literally went, this is not about physical movement here. fara að is a common verbal expression meaning to begin.

Because Icelandic often uses the dative for the person who experiences a feeling, pain, or physical condition.

So in expressions like this, the experiencer is not in the nominative:

• mér er kalt = I am cold
• honum er illt í bakinu = his back hurts / he has back pain
• mér fór að verða illt í tönninni = my tooth started to hurt

So mér is dative, and it means something like to me or for me, even though natural English usually just says my tooth hurt.

Because illt here is part of an impersonal expression, and in these expressions Icelandic very often uses the neuter singular form.

Compare:

• mér er kalt = I am cold
• henni er heitt = she is hot
• þeim er illt = they are in pain
• mér er illt í tönninni = my tooth hurts / I have pain in the tooth

So illt does not agree with tönninni. It is not describing the tooth like an ordinary adjective would. It is part of a fixed type of expression about physical sensation or discomfort.

There are two useful things to notice here.

First, í takes the dative when it expresses location or state, rather than motion into something.

Here the pain is located in the tooth, so Icelandic uses the dative:

• í tönninni = in the tooth

Second, tönninni is the definite singular form of tönn.

So:

• tönn = tooth
• tönninni = the tooth

The full phrase illt í tönninni means pain in the tooth or more naturally tooth pain / the tooth hurts.

Because Icelandic often does not use a possessive with body parts when the owner is already obvious from context.

Here, mér already tells you whose tooth it is. So í tönninni naturally means in my tooth in context.

This is very common in Icelandic:

• Mér er illt í höfðinu. = My head hurts.
• Honum er kalt á höndunum. = His hands are cold.

You can add a possessive for emphasis, but it is often unnecessary.

So in this sentence:

• mér ... í tönninni naturally means my tooth

Yes, but it is fairly regular Icelandic word order.

The sentence has three parts:

1. Tannlæknirinn sagði
   = main clause

2. að ég hefði átt að nota tannþráð oftar
   = subordinate clause after að

3. áður en mér fór að verða illt í tönninni
   = subordinate clause after áður en

In the main clause, Icelandic normally follows the usual verb-second pattern.
After conjunctions like að and áður en, you get more typical subordinate-clause order.

So the sentence is built very systematically:

• The dentist said
• that I should have used dental floss more often
• before my tooth started to hurt

Once you can spot those clause boundaries, the sentence becomes much easier to read.