nennen
to name
To give someone or something a name.
Sie haben beschlossen, ihr Kind Leo zu nennen.
They decided to name their child Leo.

to call / name
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to name
To give someone or something a name.
Sie haben beschlossen, ihr Kind Leo zu nennen.
They decided to name their child Leo.

The verb 'nennen' (to call/name) is often used with a double accusative. This means that two accusative objects appear in the sentence: the person/thing being named, and the name or label itself.
Seine Freunde nennen ihn einen Experten für alte Filme.
His friends call him an expert on old films.
In the meaning 'erwähnen' (to mention), 'nennen' (to name/mention) is often combined with words like 'Gründe' (reasons), 'Beispiele' (examples), 'Vorteile' (advantages), 'Nachteile' (disadvantages), or 'Zahlen' (figures/numbers).
Der Redner nannte drei wesentliche Vorteile des neuen Systems.
The speaker named three key advantages of the new system.
When 'nennen' (to call) is used to characterize someone, it often has a negative or derogatory meaning. The second accusative object is then usually an insult or a critical description.
Sie war so enttäuscht, dass sie ihn einen Lügner nannte.
She was so disappointed that she called him a liar.
The verb can also be used reflexively: 'sich nennen' (to call oneself). This means that a person uses a particular name or label for themselves.
Der Künstler nennt sich einfach nur „Z“.
The artist simply calls himself 'Z.'
to say
'sagen' and 'nennen' both have to do with language. 'sagen' is more general: you say a sentence, an opinion, a piece of advice, or a piece of information. 'nennen' is used when you give a name, a number, a reason, or an example. You can also call someone by a certain label. Learners often confuse the words because in both cases you pronounce something. In practical terms: if the whole content is important, 'sagen' usually fits. If it is about a name or a short detail, 'nennen' is more precise.

Mein Kollege nennt im Bus noch schnell den Ort der Veranstaltung.
My colleague quickly mentions the location of the event while we are on the bus.
Im Gespräch mit einem Freund sage ich, dass die Reparatur länger dauert.
In a conversation with a friend, I say that the repair is taking longer.