Learn Hardcore German: Hätte sie mehr Zeit gehabt, wäre sie mit ins Schwimmbad gegangen. - If she had had more time, she would have gone to the swimming pool with us.

Hätte sie mehr Zeit gehabt = Konjunktiv II of haben (+ participle): “had she had…”
wäre sie … gegangen = Konjunktiv II of sein (+ participle) with a motion verb: “she would have gone…”

This is also called Konjunktiv II der Vergangenheit or irrealis der Vergangenheit. In English: “Had she had more time, she would have gone (along) to the swimming pool.”

Where is the wenn (“if”)? Why is it missing?

German can omit wenn and mark the condition by putting the finite verb first (V1) in the conditional clause. Your sentence is equivalent to:

Wenn sie mehr Zeit gehabt hätte, (dann) wäre sie mit ins Schwimmbad gegangen.

Leaving out both wenn and dann is concise and quite idiomatic, especially in writing: Hätte …, wäre …

Why do both clauses start with a verb (Hätte, wäre)?

In the first clause, verb-first signals a conditional when wenn is omitted: Hätte sie … = “If she had …”
In the result clause, verb-first is a common, elegant pairing when dann is omitted: …, wäre sie …
If you include dann, the second clause uses normal verb-second word order: …, dann wäre sie …

Why hätte … gehabt? Isn’t that like “had had”?

Yes—just like English “had had.” It’s required here because we need the past counterfactual:

hätte … gehabt = Konjunktiv II (hypothetical) + perfect participle for past time.
Using hatte (simple past indicative) would state a fact: Sie hatte mehr Zeit = “She had more time (in reality).”
Using hätte without gehabt would be present-time: Wenn sie mehr Zeit hätte, … = “If she had more time (now), …”

Could I say Wenn sie mehr Zeit hätte, würde sie mit ins Schwimmbad gehen?

Yes, but that changes the time reference.

hätte … würde … gehen = present/future hypothetical: “If she had more time (now), she would go…”
Your original is past counterfactual: “If she had had more time (then), she would have gone…”

Why wäre … gegangen and not würde gehen or hätte gegangen?

Past unreal result uses the perfect with Konjunktiv II of the auxiliary: for gehen, that’s sein → wäre … gegangen.
würde gehen expresses a present/future hypothetical, not a past one.
hätte gegangen is ungrammatical because intransitive motion verbs like gehen, fahren, kommen take sein in the perfect, not haben.
Forms like würde gegangen sein exist but are rare/stilted here and usually convey reportedness/probability rather than a simple conditional result.

What does mit add here? Could I leave it out?

mit adds the idea “along (with them), too.”

With mit: she would have gone along with others who were going.
Without mit: she would simply have gone to the pool (no implication of accompanying someone).

Is mit the preposition that takes the dative here?

Not in this sentence. Here mit behaves like a separable adverb/prefix meaning “along/too,” combining with a motion verb (mit … gehen). It does not govern a noun.
If you wanted the preposition mit “with + person/thing,” you’d say, for example: mit ihren Freunden (dative).

Why not just use the single verb mitgehen or mitkommen?

You can say Sie wäre mitgegangen if the destination is clear from context.
When you explicitly mention the destination, German idiomatically says mit in/zu … gehen: … wäre sie mit ins Schwimmbad gegangen sounds natural.
mitkommen vs mitgehen: both mean “come/go along,” but mitkommen often frames movement toward the speaker’s location or together with the speaker, whereas mitgehen is neutral “go along.” With a specified destination like ins Schwimmbad, mit … gehen is very common.

Why is it ins Schwimmbad and not im or zum?

ins = in + das (accusative) → movement into an enclosed space: “into the swimming pool (building/area).”
im = in + dem (dative) → location: “in the swimming pool (located there).”
zum = zu + dem → movement “to/toward” a place (not necessarily inside): “to the swimming pool (as a destination).”
Here, going inside is intended, hence ins.

Is ins accusative? What exactly is being contracted?

Yes. ins is the contraction of in das (two-way preposition in with the accusative because of motion toward).
The sequence mit ins is simply the adverb mit (“along”) followed by ins (“into the…”). Don’t fuse them; it’s two words: mit ins.

Can I swap the clause order?

Yes:

Wäre sie mit ins Schwimmbad gegangen, hätte sie mehr Zeit gehabt.
With wenn: Sie wäre mit ins Schwimmbad gegangen, wenn sie mehr Zeit gehabt hätte. Punctuation and verb placement follow the same rules.

Is the comma mandatory?

Yes. You must separate the two clauses with a comma: Hätte sie … gehabt, wäre sie … gegangen.
With wenn, the comma is also mandatory: Wenn …, (dann) …

Could I use ginge instead of wäre gegangen?

Not for the past counterfactual.

ginge is Konjunktiv II simple (“would go”), typically used for present/future hypotheticals or in a formal/literary register: Wenn sie mehr Zeit hätte, ginge sie mit ins Schwimmbad.
For a past unreal result, use the perfect form: wäre gegangen = “would have gone.”

What’s the difference between hätte and hatte?

hätte = Konjunktiv II (hypothetical/irrealis): Hätte sie mehr Zeit … = “If she had more time …”
hatte = Präteritum indicative (factual past): Sie hatte mehr Zeit … = “She had more time … (in reality).”
In your sentence, hätte … gehabt signals a non-realized past condition.