Learn Dutch: Wat een giftige kikker is dat! - What a poisonous frog that is!

We use wat een because kikker (frog) is a singular, countable noun. If we were exclaiming about multiple frogs or an uncountable noun like water, we would just use wat without the een.

Why is the adjective giftige spelled with an -e at the end even though it follows een?

An adjective after een only drops the -e ending if it describes a het word. Because kikker is a de word (de kikker), the adjective keeps the -e ending.

Why is the verb is placed before dat?

This sentence follows standard Dutch V2 (verb-second) word order. The entire exclamatory phrase Wat een giftige kikker counts as the first block of information. The verb is takes the second position, pushing the subject dat to the end.

Since kikker is a de word, shouldn't it be is die instead of is dat?