This is the 179th most frequent Polish word.
wtedy
"Then" (at that time).
Wtedy is used here to introduce an event occurring immediately after the previous one in a sequence of events.
Byłem w szkole, a wtedy zaczęło padać.
I was at school, and then it started to rain.
In this sentence, 'wtedy' is used to indicate the outcome in a conditional structure.
Jeśli masz czas, wtedy pójdziemy na spacer.
If you have time, then we'll go for a walk.
Here, 'wtedy' refers to a specific time in the past.