Vädret var hemskt.

Because -et is the definite ending.

In Swedish, the is usually attached to the noun instead of being a separate word:

• väder = weather
• vädret = the weather

So Vädret var hemskt is literally something like weather-the was awful.

It is an ett word, also called a neuter noun.

You can tell from:

• the definite form vädret
• the adjective form hemskt

A small nuance: when talking about weather in general, Swedish often just uses väder without an indefinite article, but grammatically it still behaves like an ett noun.

Because adjectives in Swedish often agree with the noun.

Since väder is an ett word, the adjective takes the -t form:

• en word singular: hemsk
• ett word singular: hemskt
• plural: hemska

So compare:

• Maten var hemsk = The food was awful
• Vädret var hemskt = The weather was awful
• Dagarna var hemska = The days were awful

Var is the past tense of vara, which means to be.

So:

• är = is / are
• var = was / were

That is why the sentence is in the past: Vädret var hemskt.

Also, Swedish does not change the verb for different persons the way English does. You say:

• jag var
• du var
• han var
• vi var

The form stays the same.

Because here hemskt is not directly attached to the noun. It comes after vara and describes the subject, just like in English:

• The weather was awful

This is called a predicate adjective.

If you put the adjective before the noun, you get a different structure:

• det hemska vädret = the awful weather

So:

• Vädret var hemskt = The weather was awful
• det hemska vädret = the awful weather

A rough English-friendly guide would be:

VEH-dret var HEMSKT

A few notes:

• ä sounds roughly like the vowel in bed
• vädret has the stress on the first syllable
• var is pronounced with a long a sound in many accents
• hemskt is one syllable and has a heavy consonant cluster at the end

You do not need to make it sound exactly like English spelling suggests. The important thing at first is to hear and copy native audio.

Yes. That would simply change the tense.

• Vädret var hemskt = The weather was awful
• Vädret är hemskt = The weather is awful

So the grammar is the same; only the time changes.

Yes, but it is less specific.

Det var hemskt means It was awful, and the listener has to understand from context what it refers to.

If you want to say specifically that the weather was awful, Vädret var hemskt is clearer.

Another common Swedish way to express a similar idea is:

• Det var hemskt väder = It was awful weather / The weather was awful

Yes. Hemskt is fairly strong and usually means something like awful, terrible, or horrible.

If you want something milder, Swedish might use words like:

• dåligt = bad
• inte så bra = not so good

So Vädret var hemskt sounds stronger than just saying the weather was bad.