zunehmen
to gain weight
To gain body weight; to become heavier.
Ich habe im Urlaub leider drei Kilo zugenommen.
Unfortunately, I gained three kilos while on vacation.

to gain weight / increase
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to gain weight
To gain body weight; to become heavier.
Ich habe im Urlaub leider drei Kilo zugenommen.
Unfortunately, I gained three kilos while on vacation.

'Zunehmen' (to gain weight/increase) is a separable verb. The prefix 'zu-' goes at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Der Wind nimmt am Abend stark zu.
The wind increases strongly in the evening.
The perfect tense of 'zunehmen' (to gain weight/increase) is always formed with the auxiliary verb 'haben' (to have). The past participle is 'zugenommen'.
Die Bevölkerung der Stadt hat im letzten Jahr zugenommen.
The city's population increased last year.
When referring to body weight, 'zunehmen' (to gain weight) is often combined with quantities or the word 'Gewicht' (weight). Typical phrases are 'an Gewicht zunehmen' (to gain weight) or 'ein paar Kilo zunehmen' (to gain a few kilos).
Er hat durch den neuen Job viel an Gewicht zugenommen.
He gained a lot of weight because of the new job.
You can recognize this meaning by the fact that the subject is usually an abstract noun. It describes an increase in something immaterial such as strength, quantity, or intensity.
Die Spannung im Raum nahm spürbar zu.
The tension in the room noticeably increased.
to fall
'fallen' and 'zunehmen' are opposites when it comes to amounts, values, intensity, or a level. 'fallen' means that something becomes less or goes down. 'zunehmen' means that something becomes more or stronger. Both are often used with numbers, problems, noise, traffic, or interest. Important: with body weight, you normally say 'jemand nimmt zu' (someone gains weight), but not 'jemand fällt'. In that case, 'das Gewicht fällt' (the weight falls) would be possible, not the person.

Am Nachmittag nimmt der Verkehr vor der Schule stark zu.
In the afternoon, traffic in front of the school increases sharply.
Nach dem Feiertag fallen die Besucherzahlen im Museum wieder.
After the public holiday, visitor numbers at the museum fall again.