How do I know when to use either ‘ein’ or ‘einen’ in a sentence?

See a translation

"Einen" is used in a sentence when a masculine word becomes the object of a sentence whilst "ein" is used for masculine words when they are the subject of a sentence. Note that "ein" is used for both das (neuter) and der (masculine) nouns whilst "einen" is only ever used for masculine nouns.

So, for example, if I were to say "ein Mann isst einen Apfel" it would mean "a man eats an apple", as the "einen" indicates that the apple is the object of this sentence.

However, if I were to instead write "einen Mann isst ein Apfel" it would mean the opposite, that the man is being eaten by the apple, as the "einen" here indicates that it is the man that is the object of this sentence (the one being eaten).

When something is the subject of a sentence in German it is known as being in the nominative case (basically der, die, das or ein, eine, ein). When something is the object of a sentence it is known as being in the accusative case. I have attached a table of German cases to this answer. I hope it helps!