Learn Dutch: Waarom heb je dat blog gedeletet? - Why did you delete that blog?

The rule for regular Dutch past participles is ge- + stem + -t or -d. For English loanwords, the stem is based on the English spelling, which is delete. Since the stem ends in a voiceless "t" sound, the 't Kofschip rule dictates we add a -t. So, the formula is ge- + delete + -t, resulting in gedeletet. The silent English "e" is kept so the vowel sound doesn't change.

How do you actually pronounce gedeletet?

You pronounce it as ge-die-LIET. The ge- prefix is pronounced with the standard Dutch unstressed "e" (like the "a" in "about"), but the stem is pronounced exactly like the English word "delete". The final -t you added just merges with the existing "t" sound of the stem, so you don't pronounce a double "t" or an extra syllable at the end.

Why is the present perfect (heb ... gedeletet) used here instead of the simple past?

In spoken and conversational Dutch, the present perfect is the default choice for a completed action in the past, especially when the result of that action is relevant to the present (the blog is now gone). The simple past is generally used for storytelling or written narratives.

Why does the sentence use dat blog instead of die blog?