ablehnen
to reject
To say “no” to an offer, request, or suggestion; to not accept something.
Er hat das Jobangebot leider abgelehnt.
Unfortunately, he rejected the job offer.

to reject
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to reject
To say “no” to an offer, request, or suggestion; to not accept something.
Er hat das Jobangebot leider abgelehnt.
Unfortunately, he rejected the job offer.

'Ablehnen' (to reject) is a separable verb. The prefix 'ab-' goes to the end of the sentence in a main clause. In subordinate clauses or in the infinitive with 'zu', the verb stays together.
Sie lehnte das Angebot sofort ab.
She rejected the offer immediately.
In everyday language, 'ablehnen' (to reject) is often used with concrete offers or suggestions. Typical combinations are 'ein Angebot ablehnen' (to reject an offer), 'eine Einladung ablehnen' (to decline an invitation), or 'eine Bitte ablehnen' (to reject a request).
Leider musste ich die Einladung zur Party ablehnen.
Unfortunately, I had to decline the invitation to the party.
When someone refuses to do something, the structure 'es ablehnen, etwas zu tun' (to refuse to do something) is often used. The 'es' is a placeholder here for the following infinitive clause.
Die Firma lehnte es ab, die Gehälter zu erhöhen.
The company refused to raise the salaries.
The meaning 'zurückweisen' (to reject) is mainly used in formal, legal, or administrative contexts. You rarely hear it in everyday language.
Sein Asylantrag wurde leider abgelehnt.
Unfortunately, his asylum application was rejected.
to have
'etwas gegen jemanden oder etwas haben' and 'ablehnen' both express a negative attitude. 'Etwas gegen etwas haben' sounds more personal and often less direct: you do not like something or do not agree with it. 'ablehnen' is clearer and stronger. It fits when you do not accept an offer, request, or proposal, or when you fundamentally do not think something is good. In everyday language, people often say 'Ich habe nichts dagegen' (I have nothing against it) when they agree. 'ablehnen' fits better for a clear decision or an official response.

Nach dem Unterricht lehnt Max die Einladung zum Training ab.
After class, Max turns down the invitation to training.
Die Kundin hat nichts gegen den neuen Liefertermin.
The customer has no objection to the new delivery date.