Learn Turkish: Dün hava çok güzeldi. - Yesterday the weather was very beautiful.

In Turkish, "was" or "were" isn't a separate word. Instead, it is added to the end of the noun or adjective as a suffix. Here, we add the past tense suffix -di to güzel (beautiful) to make güzeldi (was beautiful).

Why is the suffix -di instead of -dı or -dü?

The past tense suffix follows 4-way vowel harmony, meaning its vowel changes to match the word it attaches to. You look at the last vowel of the word. The last vowel in güzel is "e", so the suffix must use an "i".

The lesson says the suffix is -(y)DI. Why is there no "y" in güzeldi?

The buffer letter y is only used when attaching this suffix to a word that ends in a vowel. This stops two vowels from crashing together (for example, hasta + ydı = hastaydı). Because güzel ends in the consonant "l", we don't need the y here.

What happened to the word "the" in "the weather"?

Turkish does not have a definite article like "the". You simply say hava (which means weather or air), and "the weather" is naturally understood from the context of the sentence.