Hún snertir ennið mitt.

• Hún = she
• snertir = touches / is touching
• ennið = the forehead
• mitt = my

So grammatically, hún is the subject, snertir is the verb, and ennið mitt is the object: my forehead.

Hún is the nominative form of she. Icelandic uses the nominative for the subject of the sentence, just like English uses she instead of her in she touches...

Other forms of this pronoun would be used in other cases, for example:

• hana = her (accusative)
• henni = her (dative)

So hún is used because she is the one doing the action.

Snerta is the infinitive, meaning to touch.

In the sentence, the verb has to be conjugated to match hún (she), so it becomes snertir, the third-person singular present form:

• að snerta = to touch
• hún snertir = she touches / she is touching

It can mean either, depending on context.

Icelandic present tense often covers both:

• she touches
• she is touching

If the surrounding context does not make the timing more specific, snertir can naturally be understood either way.

It is the definite article attached to the noun.

Icelandic often puts the at the end of the word instead of using a separate word in front.

So:

• enni = forehead
• ennið = the forehead

This is very common in Icelandic.

In Icelandic, a very common way to say my X is:

definite noun + possessive

So:

• ennið mitt = my forehead
• literally, something like the forehead my

This structure is especially normal in everyday Icelandic.

Because mitt has to agree with enni.

Possessives in Icelandic change for gender, number, and case. The noun enni is neuter singular, so the correct form of my here is mitt.

Compare:

• minn = masculine
• mín = feminine
• mitt = neuter

Since enni is neuter, you need mitt.

It is in the accusative, because snerta takes a direct object in the accusative case.

So in:

• Hún snertir ennið mitt

the thing being touched, ennið mitt, is the accusative object.

Because for this noun, the nominative and accusative singular forms are the same.

That is very common with neuter nouns in Icelandic.

So:

• nominative: ennið
• accusative: ennið

The case is still accusative because of the verb, even though the form does not change.

It is possible, but ennið mitt is the more natural, neutral everyday wording for my forehead.

Using mitt enni can sound more marked, contrastive, or stylistically special. For example, it can fit better if you are emphasizing my in contrast with someone else’s:

• ekki þitt enni, heldur mitt enni = not your forehead, but my forehead

For a simple statement, ennið mitt is the safer choice.

This sentence has the basic, straightforward order:

subject + verb + object

• Hún = subject
• snertir = verb
• ennið mitt = object

That is a very normal Icelandic sentence pattern.

However, Icelandic also has a strong verb-second tendency, so if another element comes first, the verb usually still stays in second position. For example:

• Í dag snertir hún ennið mitt = Today she touches/is touching my forehead

So the sentence you were given is simple and standard, not unusual.