Learn Hardcore Dutch: Na het sporten waste ik mijn handen grondig met die zeep. - After exercising I washed my hands thoroughly with that soap.

Here sporten is a nominalized infinitive (a gerund). Dutch uses het + infinitive as a noun to refer to the activity: het sporten = the act of exercising. Na (after) requires an object, so you say Na het sporten (After exercising), not just Na sporten.

Why is waste used instead of wast or ik was?

Waste is the simple past of wassen. The conjugation is: • Present: ik was / jij wast / hij wast / wij wassen
• Past: ik waste / jij waste / wij wasten
Because the sentence describes a past action, you use waste. Using ik was here would be ambiguous with past of zijn (to be).

Can I use the perfect tense instead of the simple past here?

Yes. In everyday Dutch the perfect is more frequent. You’d say: Na het sporten heb ik mijn handen grondig met die zeep gewassen.
You still invert the subject and auxiliary when Na het sporten is fronted: Na het sporten heb ik … gewassen.

How does word order work when the sentence starts with Na het sporten?

Dutch follows the V2 rule. If you put a time adverbial first (like Na het sporten), the finite verb goes second and the subject third: 1) Na het sporten (time)
2) waste (finite verb)
3) ik (subject)
4) rest of sentence…
If you began with the subject, you’d say Ik waste na het sporten mijn handen…

Why is mijn handen plural and not singular?

You normally wash both hands, so you use the plural handen. If you meant only one hand, you’d specify mijn rechterhand or mijn linkerhand.

What is the difference between met die zeep, met de zeep and met deze zeep?

• de zeep = the soap (definite article, general reference)
• die zeep = that soap (demonstrative, pointing to a particular soap)
• deze zeep = this soap (demonstrative, close to the speaker)
Here met die zeep means “with that specific soap” previously mentioned or indicated.

Why is grondig placed after mijn handen and before met die zeep?

Grondig (thoroughly) is an adverb modifying waste. In a Dutch main clause, adverbs generally follow the direct object. The typical order is: Time – Verb – Subject – Object – Adverb – Prepositional phrase.
So grondig comes right after mijn handen.

Why isn't there a reflexive pronoun in “waste ik mijn handen”?

Zich wassen means washing oneself (the whole body). When you wash a specific part (e.g. your hands), Dutch treats wassen as a regular transitive verb: you wash something. Hence ik waste mijn handen, not ik waste me mijn handen.

Is this sentence formal or informal? Would a native speaker actually use the simple past here?

The simple past (waste) is grammatically correct but appears more in writing or formal contexts. In spoken Dutch, most people say the perfect: Na het sporten heb ik mijn handen grondig met die zeep gewassen.
Both versions are understood.