Multi-National Operations and Inter-corporate investments

I am a little confused about how to deal with investments in foreign companies.

I am fully well aware of the Current Rate/Temporal method but I am a little confused if we are only supposed to apply it when we have control of the company?

What do we do if we have significant influence or if we are only passive investors?

Account for foreign currency gains and losses in your income statement and/or OCI (direct to equity) as applicable.