Hún finnur kjól sem hún getur hneppt sjálf, en rennilásinn á gamla kjólnum er of stífur.

Because kjól is the accusative singular of kjóll.

• kjóll = nominative singular, the basic dictionary form
• kjól = accusative singular

In this sentence, kjól is the direct object of finnur (finds), so accusative is used:

• Hún finnur kjól = She finds a dress

Sem introduces a relative clause. Here it means that, which, or sometimes just nothing in natural English.

• kjól sem hún getur hneppt sjálf
• literally: a dress that she can button herself

A useful point: sem does not change for gender, number, or case.

Because sem starts a subordinate/relative clause, and Icelandic word order is different there from main-clause word order.

In a main clause, Icelandic often uses verb-second order:

• Hún finnur kjól.

But after sem, the subject usually comes before the finite verb:

• sem hún getur hneppt sjálf

So this order is normal for a relative clause.

After geta (can / be able to), Icelandic commonly uses the form called sagnbót. For hneppa, that form is hneppt.

So:

• geta hneppt = can button / can fasten

This is just the normal pattern to learn with geta:

• geta gert = can do
• geta séð = can see
• geta hneppt = can button/fasten

Hneppa means to fasten, especially to button up clothing. It can also be used more generally for fastening something with a clasp, snap, or similar closure.

So in this sentence, it suggests a dress that she can fasten by herself, probably with buttons or a similar front fastening.

Sjálf means herself here, and it agrees with hún.

Since hún refers to a feminine singular person, the matching form is:

• sjálf = feminine singular

So:

• hún getur hneppt sjálf = she can button it herself

Compare:

• hann ... sjálfur = he ... himself
• það ... sjálft = it ... itself

The ending -inn is the suffixed definite article, so rennilásinn means the zipper.

• rennilás = a zipper
• rennilásinn = the zipper

It is definite because the sentence is talking about a specific zipper: the zipper on the old dress.

Because the preposition á can take different cases depending on meaning.

Here it means on in a location sense, so it takes the dative:

• á gamla kjólnum = on the old dress

Forms here:

• kjóll → kjólnum = dative singular definite
• gamall → gamla = matching adjective form

A useful contrast:

• á kjólnum = on the dress (location, dative)
• á kjólinn = onto the dress (movement, accusative)

Because they refer to different kinds of reference.

• kjól = a dress, not yet identified, just some dress she finds
• gamla kjólnum = the old dress, a specific known dress

So Icelandic is making the same distinction English makes between:

• a dress
• the old dress

Stífur agrees with rennilásinn, which is masculine singular nominative.

Even though stífur comes after er, it still agrees with the subject:

• rennilásinn = masculine singular
• therefore stífur = masculine singular

Also, of here means too, not English of:

• of stífur = too stiff