Learn Spanish (Latin America): Aceptar lo malo es necesario en la vida.

Lo is a special neuter article. When you put lo in front of a masculine singular adjective like malo, it turns that adjective into an abstract idea. Instead of talking about a specific bad person or a specific bad object (el malo), lo malo means 'the bad', 'the bad part', or 'bad things in general'.

Why does the sentence start with the infinitive Aceptar? The English translation says 'Accepting'.

In Spanish, when an action is the subject of a sentence, you must use the infinitive form of the verb (like aceptar). You cannot use the -ando or -iendo forms here, even though English uses the '-ing' form ('Accepting') for the exact same idea.

Why does it say en la vida if the English translation is just 'in life'?

In Spanish, you generally need to use a definite article (el, la, los, las) when talking about abstract concepts or things in a general, universal sense. Because the sentence is talking about 'life' in general, it requires la vida.

Why is necesario in its masculine singular form?

Necesario is describing the entire action of the sentence (Aceptar lo malo). Whenever an adjective is used to describe an infinitive verb or a general concept rather than a specific masculine or feminine noun, it defaults to the masculine singular form.