1) If I take a shower, I will go to the park. This may or may not happen. If I don't take a shower, then I won't go to the park. Maybe you'll be at the park. Maybe you won't be at the park. 2) When I take a shower, I will go to the park. You are taking a shower, and you are going to the park. You'll be at the park.

If I take a shower, I will go to the park. IF makes the sentence conditional. which means in order for something to happen (( going to the park )) you need the condition to happen (( take a shower )) if the condition doesn`t happen, the action won`t When I take a shower, I will go to the park. you already decided that you will take a shower then will go to the park.

They're not the same. If - もしシャワーを浴びるならば、公園に行きます。 When - シャワーを浴びるとき、公園に行きます。 So basically, "if" means 「ならば」while the English "when" would be used in similar situations as the Japanese 「とき」. I'm not sure whether I managed to write the Japanese example sentences grammatically correct, but I hope I managed to get my point across. Otherwise, I hope someone else's answer will help you! (^^)

Also it may be worth pointing out that, "When I take a shower...." is more properly expressed in this context by saying, "When I have taken a shower, I will go to the park"