Hän ottaa vielä yhden viipaleen leipää, koska hän on nälkäinen.

Yes—ottaa literally means to take, but in a sentence like this it often translates more naturally as have, take, or help oneself to.

So Hän ottaa vielä yhden viipaleen leipää can mean:

• He/She takes one more slice of bread
• He/She has another slice of bread

The exact English wording depends on context.

Vielä has several meanings in Finnish, including:

• still
• yet
• even
• more / another

Here, in vielä yhden, it means one more or another.

So:

• vielä yhden viipaleen = one more slice

This is a very common Finnish pattern:

• vielä yksi = one more
• vielä vähän = a little more
• vielä kahvia = some more coffee

Because this phrase is the object of the verb ottaa, and in this sentence it is a complete, finished object: the person is taking one whole slice.

So Finnish uses the total object form:

• yksi viipale = one slice as a basic dictionary-style phrase
• ottaa yhden viipaleen = to take one slice

The forms yhden and viipaleen are the inflected forms used here.

A helpful contrast:

• Hän ottaa yhden viipaleen. = He/She takes one whole slice.
• Hän ottaa viipaletta. = He/She takes some slice / part of a slice.
  This sounds odd here, but it shows the grammatical difference.

Because viipale leipää is a very common Finnish structure where the second noun names the substance or whole thing that the piece comes from.

So:

• viipale leipää = a slice of bread
• pala kakkua = a piece of cake
• lasi vettä = a glass of water
• kuppi kahvia = a cup of coffee

In these expressions, the second noun is usually in the partitive:

• leipää
• kakkua
• vettä
• kahvia

So yhden viipaleen leipää literally works like one slice of bread.

Because yksi is being inflected.

Basic form:

• yksi = one

In this sentence, it appears in the form yhden because it is part of the object phrase:

• yhden viipaleen

This is just normal inflection. Finnish words change form depending on their role in the sentence.

Some common forms of yksi:

• yksi = one
• yhden = of one / one as a total object form
• yhtä = one in the partitive

It can mean either.

Finnish hän is gender-neutral:

• hän = he or she

So without more context, you cannot tell the person’s gender from this pronoun alone.

In everyday spoken Finnish, people very often use se instead of hän when talking about a person, but in standard written Finnish hän is normal.

Because koska hän on nälkäinen is a subordinate clause.

Finnish normally uses a comma to separate:

• the main clause: Hän ottaa vielä yhden viipaleen leipää
• the subordinate clause: koska hän on nälkäinen

So the comma is standard punctuation here.

Yes. Koska can mean:

• because
• when

The context tells you which meaning is intended.

In this sentence, it clearly means because:

• koska hän on nälkäinen = because he/she is hungry

Examples:

• Tulin, koska pyysit. = I came because you asked.
• Koska olin lapsi, asuin Turussa. = When I was a child, I lived in Turku.

So learners need to pay attention to context.

It is correct, but it is not the only common way.

Finnish can express hungry in a few ways:

• hän on nälkäinen = he/she is hungry
• hänellä on nälkä = he/she is hungry, literally he/she has hunger

Both are correct. Many learners hear hänellä on nälkä very often in everyday Finnish.

Nälkäinen is an adjective, like hungry in English.
So this sentence is perfectly grammatical and natural.

Because it is a predicate adjective after the verb olla (to be).

In a simple sentence like:

• Hän on nälkäinen

the adjective is in the basic singular form, because it describes the subject hän.

This is similar to:

• Hän on väsynyt. = He/She is tired.
• Hän on iloinen. = He/She is happy.

So nälkäinen is just the normal form used after on here.

Sometimes yes, depending on style and context.

You may also see:

• Hän ottaa vielä yhden viipaleen leipää, koska on nälkäinen.

That is understandable and natural. The subject of the koska clause is understood to be the same person.

However, repeating hän:

• koska hän on nälkäinen

makes the sentence a little clearer and more explicit, especially in careful written language or learner examples.

They are similar, but not always exactly the same.

• vielä yhden viipaleen = one more slice
• toisen viipaleen = a second slice / another slice

In this sentence, vielä yhden emphasizes an additional one. It suggests the person already had some bread and is taking more.

So:

• Hän ottaa vielä yhden viipaleen leipää = He/She takes one more slice of bread.

That is a very natural way to express another one in Finnish.