eintauchen
to dive in
To move oneself or something completely below the surface of a liquid, usually water.
Er liebte es, im Sommer in den kühlen See einzutauchen.
He loved diving into the cool lake in summer.

to dive in / immerse
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to dive in
To move oneself or something completely below the surface of a liquid, usually water.
Er liebte es, im Sommer in den kühlen See einzutauchen.
He loved diving into the cool lake in summer.

'Eintauchen' is a separable verb. The prefix 'ein-' comes at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Ich tauche nur kurz ins kalte Wasser ein.
I’m only dipping briefly into the cold water.
The perfect tense is formed with 'sein' when a person themselves dives into or immerses themselves in something, literally or figuratively. It is formed with 'haben' when a person dips an object into a liquid.
Sie ist mutig ins Meer eingetaucht.
She bravely dove into the sea.
The figurative meaning 'sich intensiv beschäftigen' (to engage with something intensively) is always used with the preposition 'in' (+ accusative). You can immerse yourself in a world, a story, a topic, or a culture.
Wenn ich einen guten Roman lese, tauche ich komplett in die Geschichte ein.
When I read a good novel, I completely immerse myself in the story.
to dive / dip
'tauchen' and 'eintauchen' can both mean that someone or something goes below the surface of a liquid. For the sport or profession underwater, you usually say 'tauchen': a person stays underwater for some time. 'eintauchen' puts more emphasis on the movement into something, meaning the moment when something goes into water or another liquid. With objects, both can sometimes be similar, for example when you briefly hold something in paint or sauce. They can also be confused because 'eintauchen in' is additionally used figuratively: you can 'in ein Thema eintauchen' (immerse yourself in a topic). 'tauchen' normally does not fit for that.

Vor dem Backen taucht Sofia die Kekse kurz in Schokolade ein.
Before baking, Sofia briefly dips the cookies in chocolate.
Im Urlaub taucht Max zum ersten Mal mit einer Sauerstoffflasche im Meer.
On vacation, Max dives in the sea with an oxygen tank for the first time.