sich verlieben in
to fall in love with
To develop strong romantic feelings for a person.
Er hat sich auf der Party sofort in sie verliebt.
He fell in love with her immediately at the party.

to fall in love
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to fall in love with
To develop strong romantic feelings for a person.
Er hat sich auf der Party sofort in sie verliebt.
He fell in love with her immediately at the party.

'Verlieben' (to fall in love) is a reflexive verb. This means it is always used with a reflexive pronoun (mich, dich, sich, uns, euch, sich) that refers back to the subject of the sentence.
Ich habe mich im Urlaub verliebt.
I fell in love while on vacation.
The verb 'sich verlieben' (to fall in love) is always used with the preposition 'in' and the accusative. You fall in love 'in' a person or a thing.
Sie hat sich in ihren neuen Kollegen verliebt.
She fell in love with her new colleague.
The verb consists of the prefix 'ver-' and the verb 'lieben' (to love). The prefix 'ver-' often describes the beginning of a process here. 'Sich verlieben' (to fall in love) is therefore the process in which you begin to love someone.
Ich merke, wie ich mich langsam in dich verliebe.
I notice how I am slowly falling in love with you.
To express the intensity of the feelings, 'verlieben' (to fall in love) is often combined with expressions like 'Hals über Kopf' (head over heels), 'unsterblich' (immortally), or 'bis über beide Ohren' (up to both ears, i.e. completely).
Er hat sich Hals über Kopf in sie verliebt und will sie heiraten.
He fell head over heels in love with her and wants to marry her.
to love
'lieben' and 'sich verlieben' both have to do with strong feelings. 'lieben' describes a feeling that is already there: you love a person, your family, or even something like music or food. 'Sich verlieben in' describes the beginning of romantic feelings for a person. So it is a process or a moment in which love develops. Learners often confuse the words because both can sound romantic. The important point is: you fall in love with someone, but afterward you may love that person.

Lena hat sich im Urlaub in einen netten Mann verliebt.
Lena fell in love with a nice man while on vacation.
Ich liebe meine Familie und verbringe gern Zeit mit ihr.
I love my family and enjoy spending time with them.