je or u
Being too polite in the Netherlands is its own kind of mistake.
Dutch has a casual 'you' (je / jij) and a formal 'you' (u), like French tu and vous.
Coming from most other cultures your instinct will be to use u to be respectful. In the Netherlands that instinct is wrong more often than it is right.
At work everyone is je — including your boss and the director. Save u for strangers who are clearly older, and for official letters.
Heb je even?
Do you have a minute?
a colleague, any level
Wat vind jij ervan?
What do YOU think?
jij is the stressed version of je
Kunt u mij helpen?
Could you help me?
a stranger behind a counter
Geachte heer Jansen,
Dear Mr Jansen,
a formal letter
watch out
Using u with a colleague does not sound respectful. It sounds distant, or slightly sarcastic, like you are keeping them at arm's length.
Easiest approach: copy the other person. If they say je to you, say je back straight away.