Learn Hardcore Dutch: Binnenin de tas ligt mijn sleutel.

Dutch main clauses are verb-second (V2). Because the sentence starts with a location phrase (Binnenin de tas), the finite verb (ligt) must be the second element, and the subject (mijn sleutel) follows it. The neutral order without fronting is: Mijn sleutel ligt binnenin de tas. Both are correct; starting with the location emphasizes where the key is.

Can I put the subject first instead?

Yes: Mijn sleutel ligt binnenin de tas. That’s the most neutral order. Binnenin de tas ligt mijn sleutel puts extra focus on the location.

Should I use ligt or zit for a key in a bag?

Both occur, with a nuance:

… zit … is very common for things inside containers: Mijn sleutel zit in de tas.
… ligt … fits objects lying flat/on a surface: Mijn sleutel ligt in de tas. Your sentence with ligt is fine; many speakers would also say zit.

Can I say Mijn sleutel is in de tas instead of using a posture verb?

It’s understandable, but Dutch strongly prefers posture verbs (liggen, zitten, staan) for locations. More idiomatic: Mijn sleutel zit/ligt in de tas. Use is mainly with temporary presence in places like Hij is in het kantoor.

Do I need binnenin, or is in de tas enough?

In de tas is the everyday choice. Binnenin (de tas) adds the nuance of being deeper inside (not near the opening) or puts extra emphasis on the interior. All of these are possible, with slightly different focus:

Mijn sleutel zit in de tas (neutral)
Mijn sleutel zit binnenin de tas (emphasizes deep inside)
Aan de binnenkant van de tas zit… (on the inside surface)

Is binnenin one word or two? What about binnen in?

Write it as one word here: binnenin. You will also see binnen in de tas in real-life usage (treating binnen as an adverb modifying in), but binnenin is the standard compact form. Do not say binnen de tas for physical location; use in de tas or binnenin de tas.

What’s the difference between binnenin, binnen, and van binnen?

binnenin (de tas): inside, often with a sense of deeper inside.
binnen on its own means “indoors/inside” in general: Ik ben binnen.
van binnen means “from/on the inside” (physical or figurative): De tas is vies van binnen; Ik ben blij van binnen.

Why de tas and not het tas? And what about sleutel?

Both tas and sleutel are common-gender nouns, so they take de: de tas, de sleutel. With a possessive, you drop the article: mijn sleutel, not de mijn sleutel. Indefinite forms: een tas, een sleutel.

Can I use the dummy er here?

Use er primarily with an indefinite subject: Er ligt een sleutel in de tas. With a definite/possessive subject, avoid it: Er ligt mijn sleutel in de tas is odd. Say Mijn sleutel ligt in de tas.

How do I replace de tas with a pronoun?

Use R-pronouns:

erin = in it: Mijn sleutel zit erin.
hierin = in this; daarin = in that: Mijn sleutel zit hierin/daarin.

How do I turn this into a question?

Yes/no: Ligt mijn sleutel binnenin de tas? (or Zit …)
Wh-: Waar ligt mijn sleutel? — Binnenin de tas.
Also: In welke tas ligt mijn sleutel?

What happens in a subordinate clause?

The finite verb goes to the end: Ik denk dat mijn sleutel binnenin de tas ligt. Fronting no longer applies inside the clause.

What changes if it’s plural?

The verb agrees in plural: Binnenin de tas liggen mijn sleutels. Neutral order: Mijn sleutels liggen binnenin de tas.

Is binnenin formal or unusual?

It’s acceptable but a bit more emphatic/literary than plain in. In everyday speech, you’ll most often hear (zit/ligt) in de tas unless the speaker wants to stress “deep inside.”

Any pronunciation tips?

binnenin: stress on the last part; the g-sound isn’t present here.
ligt: the g is a guttural , like the ch in Scottish “loch”.
sleutel: the eu is like French “deux”; final syllable has a schwa: -tel.
tas: short a, like tahs.

Is tas always “bag”? What about zak?

Netherlands: tas = bag; zak often = pocket (e.g., broekzak).
Belgium: zak is commonly “bag,” and handtas is “handbag/purse.”
Your sentence uses tas in the general “bag” sense, which is standard in NL.