1. 为什么 why

"为什么" (wèi shén me) means "why". "为什么" is usually put at the beginning of a sentence. The sentence which begins with "为什么" can be ended with a question mark, it can also be ended with "呢" (ne)＋ "？" or "呀" (ya) ＋ "？".

2. 这周四（你omitted）能来参加…… Can you come to ... this Thursday?

In "这周四（你omitted）能来参加……" [zhè zhōu sì (nǐ) néng lái cān jiā], the subject "你" (nǐ) is omitted because it is assumed. "来" (lái) can also be omitted, but it is added for emphasis to mean "participate" as opposed to simply "coming to participate". It is commonly said in everyday spoken Chinese.