Which "if" is appropriate for what situations
假如 Jiǎrú - if
Generally used. In most case, it's for something that wouldn't happen.
要是 Yàoshi - if
Mostly used in 最近雾霾这么厉害，要是我离开北京就好了 - It'd be great if I had left Beijing! The air pollution is so heavy here.
若是 Ruòshì - if
Literally used, similar to 要是
如果 Rúguǒ - if / in case / in the event that
Generally used.
倘 Tǎng - if / supposing / in case
Almost only used in literature works.
倘若 Tǎngruò - provided that / supposing that / if
Mostly used in love letters or lyrics.
假若 Jiǎruò - if / supposing / in case
Seldom used. A bit like 假如
假使 Jiǎshǐ - if / in case / suppose / given ...
Seldom used.