Learn Romanian: Astăzi este cald afară. - Today it is warm outside.

In Romanian, you do not need a filler pronoun like "it" for weather or general statements. You just use the verb este (is) directly. So, este cald literally translates to "is warm", which naturally means "it is warm" to a Romanian speaker.

What is the difference between astăzi and azi?

They mean exactly the same thing! Azi is simply the shorter, slightly more casual version of astăzi. You can use them interchangeably in everyday conversation.

Does astăzi have to go at the beginning of the sentence?

No, Romanian word order is quite flexible. Saying Este cald afară astăzi is also completely correct and sounds natural. Putting astăzi at the front just slightly emphasizes the fact that it is warm today in particular.

How would I use this exact sentence structure to say it is cold outside?

You can simply replace cald with another weather word. For example, Astăzi este frig afară means "Today it is cold outside."