kyou no tenki ha hidoi desu.

Here it is read きょう, meaning today.

This word is a very common vocabulary item, and its reading is a little irregular if you are trying to guess it only from the kanji. So it is best to memorize 今日 = きょう as a full word.

の links one noun to another.

In 今日の天気, it means something like today’s weather or the weather of today.

So the structure is:

今日 = today
の = linking particle
天気 = weather

Together: 今日の天気 = today’s weather

When は is used as a particle, it is pronounced wa.

This is a standard feature of modern Japanese spelling. The character is still written は, but in this grammar use, you say wa.

So:

天気は is pronounced てんき は in spelling, but actually said as てんきわ

は marks the topic of the sentence.

So 天気は means as for the weather or speaking of the weather.

The sentence structure is basically:

今日の天気 = today’s weather
は = topic marker
ひどいです = is awful

A natural literal breakdown is:

As for today’s weather, it is awful.

Japanese often does not use explicit subject words like it when the meaning is already clear.

In English, you usually need something like it or a full subject. In Japanese, once the topic is given with は, the rest of the sentence can simply describe it.

So 今日の天気はひどいです is completely normal without adding anything equivalent to English it.

ひどい is an い-adjective.

That means it behaves like a Japanese adjective ending in い. It directly describes the topic:

ひどい = awful / terrible / severe

In this sentence, it is the main description of 今日の天気.

Because ひどい is an い-adjective, and the い is part of the word. You cannot remove it in the present affirmative form.

So:

ひどい = plain form
ひどいです = polite form

The です here adds politeness. It does not replace the い.

So ひどです is incorrect.

No, it is not required, but it changes the politeness level.

Compare:

今日の天気はひどい。 = plain / casual
今日の天気はひどいです。 = polite

If you are speaking politely, especially in class, with strangers, or in standard textbook Japanese, です is the normal choice.

Using は makes today’s weather the topic and presents a comment about it.

今日の天気はひどいです。
= As for today’s weather, it’s awful.

If you used が, the nuance would shift more toward identifying or emphasizing the subject:

今日の天気がひどいです。

That is less natural in many normal situations unless you specifically want to emphasize the weather as the thing that is awful.

For a learner, は is the most straightforward and natural choice in this sentence.

Yes, that is also natural, but the structure is a little different.

今日は天気がひどいです means something like:

As for today, the weather is awful.

Here:

今日 is the topic
天気が is the subject inside the comment

Compare the nuance:

今日の天気はひどいです
→ The topic is today’s weather

今日は天気がひどいです
→ The topic is today, and the comment is that the weather is awful

Both are good Japanese, but they organize the information differently.

No. Standard Japanese is normally written without spaces:

今日の天気はひどいです。

The spaces in your example are just there to help learners see the parts more clearly.

Japanese word order is more flexible than English, but you cannot move things around freely without changing the structure or emphasis.

This sentence follows a very common pattern:

noun + の + noun + は + adjective + です

If you move ひどい in front of 天気, it becomes a different structure:

ひどい天気 = awful weather

So the original sentence is not just random word order; it has a clear grammar pattern.