Orðabókin sem ég nota er góð.

Because -in is the definite article attached to the noun.

• orðabók = dictionary
• orðabókin = the dictionary

In Icelandic, the is usually not a separate word like in English. It is very often added to the end of the noun.

Also, orðabók is a feminine noun, and -in is the nominative singular definite ending here.

You generally have to learn the gender of each noun as part of the word.

In this sentence, you can tell it is feminine because of the forms around it:

• orðabókin has a feminine definite form
• góð is the feminine singular form of góður (good)

So the adjective is agreeing with orðabókin, which shows that the noun is feminine.

Sem introduces a relative clause. It works like that, which, or who in English, depending on context.

So in:

• Orðabókin sem ég nota...

the word sem connects the dictionary with the extra information I use.

It is the word that links the noun to the clause describing it.

Because sem already does that job.

In the clause:

• sem ég nota

the meaning is essentially that I use or which I use.

The noun being described, orðabókin, is understood as the object of nota. So Icelandic does not need an extra object word there.

You can think of it like this:

• orðabókin = the thing being talked about
• sem ég nota = the clause describing it

So the full structure is the dictionary [that] I use.

Because inside this relative clause, ég is the subject and nota is the verb, so normal subject-verb order is used:

• ég nota = I use

Icelandic often has verb-second word order in main clauses, but subordinate clauses, including relative clauses with sem, usually do not use that same inversion pattern.

So:

• main clause: Orðabókin ... er góð
• relative clause: sem ég nota

That is why ég comes before nota.

Nota is the present tense form meaning use with the subject ég.

In modern Icelandic, many verbs do not change much across persons in the present tense, so:

• ég nota = I use
• þú notar = you use
• hann/hún notar = he/she uses

Here, since the subject is ég, the correct form is nota.

In the full sentence, orðabókin is in the nominative singular because it is the subject of er góð.

The main clause is:

• Orðabókin ... er góð
  = The dictionary ... is good

Since the dictionary is the thing that is good, it is the subject, so nominative makes sense.

Inside the relative clause, that same noun is understood as the object of nota, but the noun itself stays outside the clause in its role in the main clause.

Because adjectives in Icelandic agree with the noun they describe in gender, number, and case.

Here, orðabókin is:

• feminine
• singular
• nominative

So the adjective must also be:

• feminine
• singular
• nominative

That gives:

• masculine: góður
• feminine: góð
• neuter: gott

So góð is the correct form.

Usually no. In Icelandic, sem is normally kept in relative clauses.

English often allows:

• the dictionary I use

instead of:

• the dictionary that I use

But Icelandic generally prefers:

• orðabókin sem ég nota

So learners should normally include sem.

Yes.

• er = is
• góð = good (feminine singular)

This is a very common structure in Icelandic:

• noun + vera (to be) + adjective

For example:

• Bókin er góð. = The book is good.
• Myndin er góð. = The film is good.

Here the relative clause simply adds more information to the subject:

• Orðabókin sem ég nota er góð.

In many contexts, yes. The Icelandic present tense can often cover both a simple present and a present-progressive idea, depending on context.

So ég nota most directly means I use, but in natural translation it can sometimes correspond to:

• I use
• I am using

depending on what is meant.

However, if someone specifically wants to emphasize an ongoing action right now, Icelandic may use other expressions.

A rough guide is:

• orðabókin ≈ OR-tha-bo-kin

A few useful pronunciation notes:

• ð in Icelandic is often like the th in this
• ó is a long vowel, roughly like o in go
• stress in Icelandic is usually on the first syllable

So the stress falls on ORÐA-.

This is only an approximation, but it helps as a starting point.