jemandem zusetzen
to affect
To put a strong strain on someone, cause them difficulties, or affect them strongly emotionally or physically.
Die ständige Kritik hat ihm sichtlich zugesetzt.
The constant criticism visibly affected him.

to affect / add
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to affect
To put a strong strain on someone, cause them difficulties, or affect them strongly emotionally or physically.
Die ständige Kritik hat ihm sichtlich zugesetzt.
The constant criticism visibly affected him.

'Zusetzen' (to affect/add) is a separable verb. In a main clause, the prefix 'zu' comes at the end of the sentence. In subordinate clauses or in the infinitive with 'zu', the verb stays together.
Der Lärm der Baustelle setzt den Anwohnern sehr zu.
The noise from the construction site is really affecting the residents.
You can recognize the meaning from the grammar. 'Jemandem zusetzen' (to affect someone, with dative) means to put strain on someone. 'Etwas zusetzen' (to add something, with accusative) means to add something. 'Geld zusetzen' (to lose money, with accusative) means to make a loss.
Die Hitze setzt mir heute wirklich zu.
The heat is really affecting me today.
When 'zusetzen' means 'belasten' (to burden/put strain on), the person who is affected is always in the dative. The thing causing the strain (e.g. stress, noise, illness) is the subject of the sentence.
Der Tod seines Hundes hat ihm schwer zugesetzt.
The death of his dog hit him hard.
In the meaning 'hinzufügen' (to add), the verb is often used in the context of cooking, baking, or chemistry. Typical things that are added are 'Salz' (salt), 'Zucker' (sugar), 'Gewürze' (spices), or 'Zusatzstoffe' (additives).
Dem Trinkwasser werden hier keine Chemikalien zugesetzt.
No chemicals are added to the drinking water here.
to lose
'verlieren' and 'zusetzen' can both mean, in the context of money, that you end up with less than you had before. 'verlieren' is the normal, general word: you can lose money in a business deal, a bet, or because of a mistake. 'zusetzen' is used more specifically for a business deal or venture when you suffer a financial loss. It sounds somewhat more formal and is often used in fixed contexts such as 'Geld zusetzen' (to lose money). If you are talking about a loss in general, 'verlieren' is more natural. If a business does not bring in enough money, 'zusetzen' can be more precise.

Mit dem kleinen Laden setzt sie jeden Monat Geld zu.
She loses money every month with the small shop.
Beim Verkauf des alten Autos verliert Paul viel Geld.
Paul loses a lot of money selling the old car.