Meine Freundin lässt häufig ihr Handy in der Schublade liegen und sucht es dann überall.

Because German uses a verb‐cluster rule when you have one finite verb plus another infinitive. Here lässt (from lassen) is the finite verb in second position, and the infinitive liegen moves to the very end. This pattern commonly appears with verbs like lassen, modals (können, müssen…) and in perfect tense (auxiliary + participle).

ihr Handy is a direct object in accusative case. You know it’s accusative because it’s what she “leaves” (she lässt something). Handy is masculine, so the definite article would be das Handy in nominative; in accusative it would stay das Handy, but with the possessive ihr it’s clear that ihr Handy is the thing being acted upon.

The preposition in can take either dative (static location) or accusative (motion into). Here it describes where the phone is left (it’s staying “in the drawer”), so it’s a location question (wo?). That requires dative: der Schublade (feminine dative).

Adverbs of frequency (häufig, oft, immer) occupy the so-called Mittelfeld. You’ll often see them
1) right after the finite verb: lässt häufig ihr Handy…
2) before the object: lässt ihr Handy häufig…
3) even at the very front for emphasis: Häufig lässt sie ihr Handy…
Just remember the finite verb must stay in second position.

In German, coordinating conjunctions like und, oder or aber do not take a comma when they simply link two main clauses with their own finite verbs and the same subject. Commas are only needed with subordinating conjunctions (weil, dass, wenn…) or to separate items in a list.

es is the accusative pronoun that replaces ihr Handy to avoid repetition. In German you generally include the pronoun even if the noun was just mentioned. Here sucht es = “searches for it.”

liegen lassen literally means “to leave lying” or “to leave behind.” It’s separable in present tense: she lässt liegen (she leaves it lying there/in the drawer).

This is a new main clause joined by und.
1) und (conjunction)
2) sucht (finite verb in second position for that clause)
3) es (object pronoun)
4) dann (temporal adverb)
5) überall (modal adverb of place)
All adverbs follow after the verb-object chunk unless you front one for emphasis.

German spelling rules say: after a short vowel you write ss, after a long vowel or diphthong you write ß. Since the vowel ä in lässt is short, it’s spelled with ss.