难怪【nánguài】

难怪 goes at the start of a clause and says you now understand something that had puzzled you. The reason usually comes first, and 难怪 introduces what it explains.

The order can also run the other way: 难怪 first, then the reason with 原来.

难怪 can stand alone as a reply: 难怪！ "That explains it!"

难怪 goes first

难怪 starts its clause, before the subject. It can't sit between the subject and the verb.

• × 他难怪这么累 → 难怪他这么累

What follows is unsurprising

What follows 难怪 is something that now makes sense. It can't introduce a surprise or a contradiction.

• × 他每天都练习，难怪考得不好 → 他每天都练习，可是考得不好

也难怪

At the end of a sentence, 也难怪 means "it's understandable": no one can be blamed for it.

怪不得 means the same and is a little more colloquial.