Prepositions with people
When a verb is followed by a preposition and you want to replace the noun with a pronoun, the rules in Dutch depend heavily on whether you are talking about a person or a thing.
For humans, you simply use the preposition followed by the object form of the personal pronoun (e.g., hem, haar, hen, jou). This works exactly like English ("about him", "on her", "at them").
Prepositions with inanimate objects
When you are referring to an inanimate object, an idea, or a situation, Dutch has a strict rule: you cannot use a preposition followed by a standard pronoun like het or ze.
Instead, you must combine the word er with the preposition. For example, instead of saying over het (about it), you must say erover (literally: "there-about"). Instead of in het (into it), you say erin. These combinations are called pronominal adverbs and are written as one word.
Some prepositions form combinations that are very common, such as ernaar (to it / at it).
Comparing people and objects
A common mistake for English speakers is to use "preposition + pronoun" for things, or "er + preposition" for people. Remember this animacy restriction: people get personal pronouns, objects get er.
Notice how the exact same preposition behaves differently depending on what is being looked at:
Whether you are evaluating a formal opponent or weighing options, always check if the noun is alive before choosing your pronoun strategy!