Aptiten är ett viktigt symtom.

Because aptiten is the definite singular form of aptit.

• aptit = appetite
• aptiten = the appetite

In Swedish, definiteness is often shown by adding an ending to the noun instead of using a separate word like the.

So:

• en aptit = an appetite
• aptiten = the appetite

Because Swedish usually puts the at the end of the noun as a suffix.

So English:

• the appetite

becomes Swedish:

• aptiten

This is very common in Swedish:

• boken = the book
• stolen = the chair
• aptiten = the appetite

Är is the present tense of vara, which means to be.

So:

• är = is / are / am depending on context

Unlike English, Swedish verbs do not change depending on the subject:

• jag är = I am
• du är = you are
• han är = he is
• de är = they are

So in this sentence, Aptiten är ... simply means The appetite is ...

Because symtom is an ett-word in Swedish, also called a neuter noun.

That means its indefinite article is:

• ett symtom = a symptom

not:

• en symtom

In Swedish, nouns belong to one of two grammatical genders:

• en-words
• ett-words

You usually have to learn the gender together with the noun.

Because adjectives in Swedish must agree with the noun they describe.

Here, viktig describes symtom, and symtom is an ett-word. In the singular indefinite with an ett-word, many adjectives add -t.

So:

• en viktig bok = an important book
• ett viktigt symtom = an important symptom

Compare:

• viktig for many en-words
• viktigt for singular ett-words

The sentence follows the normal Swedish main-clause pattern:

subject + verb + complement

So:

• Aptiten = subject
• är = verb
• ett viktigt symtom = complement

This is the same basic order as in English:

• The appetite is an important symptom

Also, Swedish main clauses usually keep the verb in the second position. Here the subject comes first, so the verb naturally comes right after it.

Because the sentence is identifying something as a symptom, not the symptom.

So:

• ett viktigt symtom = an important symptom
• det viktiga symtomet = the important symptom

In this sentence, the speaker is classifying appetite as one symptom among others, so the indefinite form is used.

Yes. Symtom is the standard modern Swedish spelling.

It is related to English symptom, but Swedish spelling has been simplified here.

So for learners, the important thing is:

• English: symptom
• Swedish: symtom

They are cognates, but the spelling is not identical.

A simple English-friendly guide is:

• aptiten ≈ ap-ti-TEN
• symtom ≈ sym-TOM

The stress is on the last syllable in both words.

A couple of extra notes:

• The y in symtom is not exactly like English y. It is a rounded vowel that English does not really have.
• The final -en in aptiten is clearly pronounced, since it is part of the definite ending.

If you want, you can think of:

• aptiten = ap-ti-TEN
• symtom = süm-TOM

That is only an approximation, but it helps.

Grammatically, aptit without the definite ending is possible in some contexts, but it would sound less natural here.

Aptiten usually refers to a specific appetite, often the one being discussed in a medical or practical situation, which makes sense in a sentence like this.

So:

• aptiten = the appetite
• aptit = appetite in a more general sense

In this sentence, aptiten is the more natural choice.