Symptomet er tydelig.

Because symptomet means the symptom, while et symptom means a symptom.

Norwegian usually marks definiteness by adding an ending to the noun:

• et symptom = a symptom
• symptomet = the symptom

The noun symptom is a neuter noun, so its definite singular ending is -et.

In Norwegian, the definite article is often attached to the end of the noun instead of standing alone as a separate word.

So:

• the symptom → symptomet
• not a separate the + symptom pattern like in English

However, Norwegian can use a separate determiner in other structures, especially when an adjective comes before the noun:

• det tydelige symptomet = the clear symptom

So in your sentence, symptomet already includes the idea of the.

Er is the present tense of å være, which means to be.

Examples:

• jeg er = I am
• du er = you are
• han er = he is
• symptomet er = the symptom is

Unlike English, Norwegian uses er for all persons in the present tense.

Because this adjective comes after er, so it is a predicative adjective.

In Norwegian, predicative adjectives agree with gender and number, but not with definiteness. That means the fact that symptomet is definite does not force tydelige.

Also, symptom is a singular neuter noun, and for adjectives ending in -ig, the neuter singular form is usually the same as the basic form:

• et tydelig symptom
• symptomet er tydelig

You use tydelige in forms such as:

• tydelige symptomer = clear symptoms
• det tydelige symptomet = the clear symptom

Often yes, but not always.

Many adjectives do add -t in the neuter singular:

• stor → stort
• klar → klart

But adjectives ending in -ig usually do not add another -t:

• viktig → et viktig spørsmål
• tydelig → et tydelig symptom

So tydelig is already the correct form here.

You learn it together with its article: et symptom.

That tells you it is neuter. From that, you can build other forms:

• et symptom = a symptom
• symptomet = the symptom

This is a good habit in Norwegian: learn nouns with en or et, not by themselves.

No. It follows a very normal pattern:

• Symptomet = subject
• er = verb
• tydelig = complement

So the sentence structure is basically the same as in English:

• The symptom is clear
• Symptomet er tydelig

Then the form changes.

Compare:

• et tydelig symptom = a clear symptom
• symptomet er tydelig = the symptom is clear
• det tydelige symptomet = the clear symptom

Notice the important difference in the definite noun phrase:

• you add det
• the adjective becomes tydelige
• the noun is still definite: symptomet

This is a very common Norwegian pattern called double definiteness.

It can often cover several nearby English meanings, depending on context:

• clear
• obvious
• evident
• distinct

So Symptomet er tydelig could sound like:

• The symptom is clear
• The symptom is obvious
• The symptom is evident

The exact best translation depends on the situation, but grammatically the Norwegian sentence stays the same.