Learn Hardcore Dutch: Het gevaar is groot in het donkere bos. - The danger is great in the dark forest.

Gevaar is a neuter noun in Dutch, so it takes the definite article het. Unfortunately there isn’t a fool-proof rule for every noun—you generally have to learn the article with each word or use a dictionary. As a guideline, diminutives (ending in -je) are always het, and many abstract nouns like het gevaar, het geluk, het leven are het, but there are exceptions.

Why is the adjective groot not grote in het gevaar is groot?

When an adjective follows a linking verb like zijn (to be), it is used predicatively and stays in its base form (no -e). If you place that same adjective before a definite noun, it becomes grote (e.g. het grote gevaar).

Why does donkere have an -e ending in het donkere bos?

That’s an attributive adjective (it directly modifies the noun before it). In Dutch, any adjective before a definite noun (with de or het) takes -e, regardless of gender or number. So donker becomes donkere in het donkere bos.

Can I move in het donkere bos to the beginning of the sentence?

Yes. Dutch is a verb-second (V2) language. If you start with a phrase, the conjugated verb still must come in the second position. For example:
In het donkere bos is het gevaar groot.

Why do we repeat het before donkere bos in the phrase in het donkere bos? Could I drop it like in English?

Dutch almost always requires the article in noun phrases, even after prepositions. You can’t say in donker bos. You need either in het bos or in een bos, and if there’s an adjective, you inflect it (e.g. in een donker bos).

What’s the difference between gevaar and gevaarlijk?

Gevaar is a noun meaning “danger.”
Gevaarlijk is an adjective meaning “dangerous.”
So you’d say het gevaar for “the danger” versus een gevaarlijk bos for “a dangerous forest.”

How would I express “the danger is greater in the dark forest”?

Form the comparative of groot, which is groter, and keep the rest of the structure:
Het gevaar is groter in het donkere bos.