“假如”用英语怎么说

If you are in BeiJing...
If you in Beijing

假如用if
if引导的是条件状语从句，必须根据语境来选择if引导的从句的时态
这里 假如你在北京可以说 If you are in Beijing,
用虚拟语气的话就说 If you were in Beijing,

assume

Assume he is in Beijing.

If(假如） you are(一般用）/were（虚拟语气时用）in(在北京介词用in,大地方用in) Beijing

If you were in Beijing

If you are in Beijing.