Learn Polish: Było tu dużo wypadków. - There were a lot of accidents here.

After words that express quantity like dużo (a lot), Polish uses the genitive case. Since we are talking about multiple accidents, we need the genitive plural form. For most masculine nouns ending in a consonant, the genitive plural ending is -ów.

The dictionary form is wypadek. Where did the second 'e' go in wypadków?

The noun wypadek has a 'fleeting e'. When you add an ending to many Polish words that end in -ek, the last e drops out to make the word smoother to pronounce. So the root wypadek shrinks to wypadk- before adding the -ów ending.

Why is the verb było (it was) in the neuter singular form, when the English translation says 'there were' (plural)?

In Polish, when the main subject is a quantity word like dużo (a lot), mało (few), or a number from five upwards, the phrase is treated grammatically as a single neuter entity ('it'). Because of this rule, the past tense verb must take the neuter singular form było, rather than a plural form.

Could I use wiele instead of dużo in this sentence?

Yes, you could absolutely say Było tu wiele wypadków. Both dużo and wiele can mean 'many' or 'a lot' when talking about countable things like accidents, and both force the noun that follows into the genitive plural.