to fall for / fail / drop in
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to fall for
Colloquial for: to let oneself be deceived by someone or something; to fall victim to a scam or trick.
Ich kann nicht glauben, dass ich auf diesen alten Trick reingefallen bin.
I can't believe I fell for that old trick.
'Reinfallen' (to fall for / fail / drop in) is a separable verb. The prefix 'rein' comes at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Er fällt immer auf dieselben Witze rein.
He always falls for the same jokes.
Since 'fallen' (to fall) describes a movement or a change of state, the perfect tense of 'reinfallen' is always formed with the auxiliary verb 'sein'.
Leider bin ich in der Fahrprüfung reingefallen.
Unfortunately, I failed the driving test.
The figurative meanings—being deceived, failing an exam, or visiting spontaneously—are all colloquial. Only the literal meaning 'in etwas hineinfallen' (to fall into something) is stylistically neutral.
Er ist auf ein gefälschtes Angebot im Internet reingefallen.
He fell for a fake offer on the internet.
When 'reinfallen' means 'getäuscht werden' (to be deceived), it is always used with the preposition 'auf' and the accusative. You fall 'auf' a trick, a scam, or a person.
Fall nicht auf seine leeren Versprechungen rein!
Don't fall for his empty promises!
In the meaning 'nicht bestehen' (to fail), 'reinfallen' is often used with the preposition 'in' and the dative. You fail 'in der Prüfung' or 'im Examen' (in the exam).
Sie hat Angst, in der mündlichen Prüfung reinzufallen.
She is afraid of failing the oral exam.
to pass / consist / exist
'bestehen' and 'reinfallen' are opposites when talking about exams. 'bestehen' means that you successfully pass an exam or test. 'reinfallen' colloquially means that you fail an exam. The words can be confused because 'reinfallen' also has other meanings, for example falling for a trick. In an exam context, however, it is clearly negative. In formal situations, people are more likely to say 'nicht bestehen' or 'durchfallen'. 'reinfallen' sounds clearly more colloquial.
Nele ist beim ersten Versuch in Mathe reingefallen.
Nele failed math on her first attempt.
Ich habe die Prüfung bestanden, weil ich jeden Abend gelernt habe.
I passed the exam because I studied every evening.