Saumurinn á skyrtunni er beinn, en það er lítil rifa neðst á henni.

A fairly literal breakdown is:

Saumurinn = the seam
á skyrtunni = on the shirt
er beinn = is straight
en = but
það er = there is
lítil rifa = a small tear / slit
neðst á henni = at the bottom of it

So the sentence is structured like:

The seam on the shirt is straight, but there is a small tear at the bottom of it.

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

• saumur = a seam
• saumurinn = the seam

Icelandic usually adds the to the end of the noun instead of using a separate word like English does.

Here, saumurinn refers to a specific seam, so the definite form is used.

This has two parts:

• skyrta = shirt
• skyrtunni = the shirt in the dative singular

The preposition á often takes the dative when it means on in a location sense.

So:

• á skyrtu = on a shirt
• á skyrtunni = on the shirt

In this sentence, the seam is located on the shirt, so á + dative is used.

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

The noun saumurinn is:

• masculine
• singular
• nominative

So the adjective also has to be masculine singular nominative:

• beinn = straight (masculine singular nominative)

Compare:

• beinn saumur = a straight seam
• bein lína = a straight line
• beint borð = a straight table / direct table-form adjective in neuter contexts

So beinn is used because it matches saumurinn.

This is the common Icelandic way to say there is / there are.

Literally, það er means it is, but in sentences like this it works like English there is:

• Það er hundur úti. = There is a dog outside.
• Það er lítil rifa... = There is a small tear...

So this is a standard existential construction.

Because rifa is a feminine noun, and the adjective has to agree with it.

• lítill = masculine
• lítil = feminine
• lítið = neuter

Since rifa is feminine singular nominative, the correct form is:

lítil rifa = a small tear

Because the sentence treats them differently in terms of information:

• saumurinn = a specific seam already identified as the seam on the shirt
• lítil rifa = an unspecified small tear, introduced as new information

So Icelandic, like English, can mix definite and indefinite nouns in the same sentence:

• Saumurinn = the seam
• lítil rifa = a small tear

If you wanted the small tear, you would use a definite form instead.

Neðst means lowest down, at the bottom, or at the lower end.

It is an adverb of position here, not an adjective modifying a noun directly.

So:

• neðst á henni = at the bottom of it
• neðst á skyrtunni = at the bottom of the shirt

It is a very natural word when describing where something is on an object.

Because this is location, not movement.

With á, Icelandic often distinguishes between:

• dative for being on something
• accusative for moving onto something

Here the tear is already located on the shirt, so the phrase uses dative:

• á henni = on it / on her in form, but here on it

If it were movement, you might see accusative instead.

Because Icelandic pronouns follow grammatical gender, not biological sex.

The noun skyrta is feminine, so when you refer back to it, you use the feminine pronoun form:

• skyrta → feminine
• henni = to/on/in her/it in the feminine dative singular

So á henni means on it, referring back to skyrtunni. It does not mean the shirt is female in a real-world sense.

Because both phrases involve the idea of something being on the shirt:

• saumurinn á skyrtunni = the seam on the shirt
• rifa neðst á henni = a tear at the bottom of it

In English, learners sometimes expect different prepositions, but Icelandic is happy to use á in both places. The exact English translation may vary a bit depending on context, but the basic spatial relationship is the same.

Rifa usually means tear, rip, slit, or small split/opening, depending on context.

In clothing, it often suggests a narrow tear or split in the fabric. So in this sentence, lítil rifa is most naturally understood as a small tear or a small slit.

The best English word depends on the picture or situation, but the Icelandic word is completely natural here.