Orðabókin er mjög gagnleg þegar ég skil ekki allt.

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

• orðabók = dictionary
• orðabókin = the dictionary

This is very common in Icelandic:

• bíll = car
• bíllinn = the car

So Orðabókin means the dictionary.

The adjective has to agree with the noun it describes in gender, number, and case.

Here, orðabókin is:

• feminine
• singular
• nominative

So the adjective also takes the feminine singular nominative form:

• masculine: gagnlegur
• feminine: gagnleg
• neuter: gagnlegt

That is why the sentence has Orðabókin er mjög gagnleg.

Mjög means very and it modifies the adjective gagnleg.

So:

• gagnleg = useful
• mjög gagnleg = very useful

It works much like English very:

• Hún er mjög góð. = She/It is very good.
• Þetta er mjög erfitt. = This is very difficult.

This is the normal Icelandic order for a simple clause:

• ég = I
• skil = understand
• ekki = not
• allt = everything

So literally: I understand not everything.

In natural English, we say I do not understand everything, but Icelandic does not need do for negation the way English does. The negative word ekki usually comes after the finite verb:

• Ég tala ekki íslensku. = I do not speak Icelandic.
• Hann kemur ekki. = He is not coming / He does not come.

Skil is the 1st person singular present tense of the verb skilja.

So:

• að skilja = to understand
• ég skil = I understand

Some present-tense forms of this verb are:

• ég skil = I understand
• þú skilur = you understand
• hann/hún/það skilur = he/she/it understands
• við skiljum = we understand

So in the sentence, ég skil ekki allt means I don’t understand everything.

In Icelandic, allt can stand on its own and mean everything.

It is the neuter singular form of allur (all), and Icelandic often uses the neuter singular for general or unspecified things.

Compare:

• allt = everything
• allt þetta = all this / everything here
• allur dagurinn = the whole day (masculine noun)
• öll bókin would not be correct here because bók is feminine and the meaning is different

So allt by itself is perfectly natural for everything.

Here þegar means when.

It introduces a time clause:

• þegar ég skil ekki allt = when I do not understand everything

Be careful, because þegar can also mean already in other contexts.

For example:

• Ég er þegar kominn. = I have already arrived.

So the meaning depends on the sentence. In your example, it is clearly the conjunction when.

In modern Icelandic, commas are not always used in exactly the same way as in English. A short subordinate clause introduced by þegar often appears without a comma in everyday writing, especially in simple sentences like this one.

So:

• Orðabókin er mjög gagnleg þegar ég skil ekki allt.

is completely normal.

You may sometimes see commas in more complex sentences, but you should not assume Icelandic punctuation follows English comma rules exactly.

Er is the present tense of vera (to be) for he/she/it and also for many singular noun subjects.

Since orðabókin is a singular noun, Icelandic uses er:

• Orðabókin er gagnleg. = The dictionary is useful.

Compare:

• Ég er = I am
• Þú ert = you are
• Hún er = she is
• Bókin er = the book is

So er simply links the subject orðabókin with the description mjög gagnleg.

Because Icelandic normally expresses the by attaching it to the noun as a suffix.

So instead of:

• the dictionary

Icelandic usually says:

• orðabókin

This is one of the biggest differences from English. Icelandic can also use a separate determiner in some special situations, but the normal everyday definite form is the attached article.

You could say something like that, but it would sound more literal and less natural in many situations.

• allt is the normal, compact way to say everything
• alla hluti means all things, which is more explicit and often less idiomatic

So in this sentence, ég skil ekki allt is the most natural choice.

It can describe both, depending on context. It means useful or helpful.

Examples:

• Orðabókin er gagnleg. = The dictionary is useful.
• Hún var mjög gagnleg. = She was very helpful.

The form changes depending on the gender of the noun or pronoun:

• gagnlegur (masculine)
• gagnleg (feminine)
• gagnlegt (neuter)

So in your sentence it is feminine because orðabók is feminine.