der Tenor
tenor
The highest natural male singing voice in music.
Die Arie ist für einen Tenor mit sehr hoher Stimme geschrieben.
The aria is written for a tenor with a very high voice.

tenor
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tenor
The highest natural male singing voice in music.
Die Arie ist für einen Tenor mit sehr hoher Stimme geschrieben.
The aria is written for a tenor with a very high voice.

To distinguish the meanings, pay attention to the context. Words like 'Oper' (opera), 'singen' (to sing), or 'Stimme' (voice) point to the musical meaning. Words like 'Diskussion' (discussion), 'Kritik' (criticism), or 'allgemein' (general) point to the figurative meaning of the main idea.
Der Tenor der Oper war weltberühmt.
The tenor of the opera was world-famous.
In the figurative meaning, 'Tenor' (tenor/gist) is often combined with 'allgemein' (general) or 'grundlegend' (basic/fundamental). Formulations with the genitive or 'von' (of/from) are also very common to clarify the reference.
Der allgemeine Tenor der Kommentare war zustimmend.
The general tenor of the comments was approving.
The meaning 'Grundgedanke' (main idea) is used mainly in written language, in the news, or in more formal discussions. It sounds more educated than everyday expressions.
Der Tenor des Artikels war, dass mehr für den Umweltschutz getan werden muss.
The tenor of the article was that more must be done for environmental protection.
In a musical context, 'Tenor' (tenor) describes either the vocal range or the singer himself. Typical adjectives here are 'lyrischer Tenor' (lyric tenor) or 'dramatischer Tenor' (dramatic tenor).
Die Rolle des Prinzen wird von einem lyrischen Tenor gesungen.
The role of the prince is sung by a lyric tenor.
tendency / trend
'die Tendenz' and 'der Tenor' can both express a general direction. 'Tendenz' is often used for developments, numbers, opinions, or behavior. 'Tenor' fits especially when talking about the main idea of a statement, text, or discussion. So you ask about the 'Tenor' when you want to know what was said at its core. 'Tendenz' is broader and can also describe a change over time.

Der Tenor der Besprechung war, dass das Projekt mehr Zeit braucht.
The tenor of the meeting was that the project needs more time.
In der Pause wurde eine klare Tendenz zu mehr Homeoffice sichtbar.
During the break, a clear trend toward more working from home became visible.