1) There are small differences in these four words that can make it difficult to differentiate them. Here are some guidelines to help:
Speak is used when one person addresses a group.
Talk suggests that two or more people are having a conversation. (Sometimes can be used interchangeably with "speak")
Say is used with words. Structure: say something to somebody.
Tell is used when giving information. Structure: tell somebody something.
2) Contact and touch can be used interchangeably in most cases, but typically, "contact" would be used as a noun and "touch" as a verb.
"The basketball player made contact with the shooter."
"The basketball player touched the shooter."
3) When is used to describe an action that caused a brief interruption or preceded or followed another action. For example:
"Bob stopped cleaning when you called him."This shows that because of your phone call, Bob had to stop what he was doing and talk to you. While describes an action that occurs at the same time as another. For example:
"Bob was cleaning while he talked to you on the phone."Bob is doing both actions at once.