This question, which is correct and why

I have recently been pointed in the right direction to assist me with my headache of studying Japanese conditional forms such as ば、たら、なら etc.

Can someone explain this in a simple way for me?
The one point remaining i can't seem to work out is using たら as a marker of time rather than 'IF'

I'm quite clear on using たら as IF.

Today i saw the example...

日本に行ったら、マンガを買います。

Before this, I thought the correct way to say this would be,

日本に行く時、マンガを買います。

Can someone please explain why and when you use たら as a 'when this happened/happen rather than using 時？

Thank you

There are many threads regarding the conditional clause in this forum.
たら vs. ば vs. と? Conditonals | Japan Forum
もし and ならば | Japan Forum
Need a word or phrase translated? | Page 286 | Japan Forum
場合(?) | Japan Forum

As I wrote somewhere in a thread linked above, the conditional clause is one of the hardest grammatical issues to grasp for non-native learners. Most textbooks use many pages for these structures. It's almost impossible to explain it in a simple way.
As for your example, たら is used there since it's "one-off situation", and とき is wrong since it means "before I go to Japan". See the following post.
時 in the following sentence | Japan Forum

It's been described to me as the various forms of "if" living on a spectrum of certainty, with ～ば at the less certain end, and ～たら at the other end, so much so that it's less "if" but more "when/once" the conditional event happens.

Thank you guys!