Learn Dutch: Een moeilijk gesprek is soms heel belangrijk. - A difficult conversation is sometimes very important.

This is because gesprek is a het-word (het gesprek). When a singular het-word is used with een (a/an), the adjective in front of it does not get an -e added to the end. If we used het instead of een, it would be het moeilijke gesprek.

Why doesn't belangrijk get an -e ending?

Belangrijk (important) describes the conversation, but it comes after the verb is, rather than directly in front of the noun. In Dutch, adjectives only ever get an -e ending when they are placed directly in front of the noun they describe.

I thought heel meant "whole". Why does it mean "very" here?

You are right that heel can mean "whole" (as in de hele wereld for "the whole world"). However, when you place heel directly in front of an adjective like belangrijk, it acts as an adverb meaning "very".

Moeilijk and belangrijk look like they have similar endings. Are they pronounced the same way?