Which preposition do I use with countries?

The use of prepositions to say you are 'in' a country is actually quite simple in French. Your two options are 'en'  (which is more common) and 'au'.

'Au' is used with masculine countries that begin with a consonant.

example: mon frère va au Pakistan

'En' is used with feminine countries...

example: nous sommes en France

...and masculine countries that begin with a vowel.

example: je vais en Afghansitan.

As long as you know the gender of the country (unless it starts with a vowel, in which case you don't even need the gender to determine the preposition), finding the right preposition is easy!

To summarise:

masc consonant: au

fem/masc vowel: en

Reminder: names of towns ALL use the preposition 'à' (no matter what the gender)

example: je vais à Paris.