sich verknallen in
to fall for
(Colloquial) To suddenly and intensely fall in love with someone. It describes a strong, often youthful crush.
Er hat sich total in die neue Mitschülerin verknallt.
He has totally fallen for the new girl in his class.

to fall for / to shoot off
Loading your saved state.
to fall for
(Colloquial) To suddenly and intensely fall in love with someone. It describes a strong, often youthful crush.
Er hat sich total in die neue Mitschülerin verknallt.
He has totally fallen for the new girl in his class.

This verb is reflexive and always needs the preposition 'in' with the accusative. So the structure is: 'sich in jemanden verknallen' (to fall for someone).
Ich glaube, ich habe mich in meine Kollegin verknallt.
I think I've fallen for my colleague.
'Sich verknallen' (to fall for someone) is very colloquial. You use it in informal conversations with friends, but not in formal situations.
Meine kleine Schwester hat sich im Urlaub total verknallt.
My little sister totally fell for someone while on vacation.
The word contains the stem 'knallen' (to make a loud bang). This emphasizes the sudden and intense nature of falling in love, as if you were struck by lightning.
Als er sie zum ersten Mal sah, hat es bei ihm sofort geknallt und er war verknallt.
When he saw her for the first time, it hit him immediately and he was smitten.
to love
'lieben' and 'sich verknallen in' both have to do with romantic feelings. 'lieben' usually describes a deep and serious feeling that can last for a long time. 'sich verknallen' is colloquial and means that you suddenly feel strongly attracted to someone. It often sounds youthful, spontaneous, and a bit infatuated. Learners may confuse the words because both have to do with being in love. The difference is: you can 'sich verknallen' quickly, but love is usually deeper and more stable.

Im Sprachkurs hat sich Max in eine neue Teilnehmerin verknallt.
Max has fallen for a new participant in the language course.
Sie liebt ihren Partner auch nach vielen gemeinsamen Jahren.
She still loves her partner after many years together.