Sub query in Entity Framework

This type of subquery can be flattened to a join, which is the way I would choose to write it here:

SQL Version:

SELECT t1.col1, t1.col2, t1.col3, ...
FROM table1 t1
INNER JOIN table2 t2
    ON t1.col1 = t2.col1
WHERE t2.col2 = 'xyz'

Linq Version:

var query =
    from t1 in context.Table1
    where t1.AssociationToTable2.Col2 == "xyz"
    select new { t1.Col1, t1.Col2, ... };

Where AssociationToTable2 is the relationship property - it does the join automatically. Or, if you don't have a relationship:

var query =
    from t1 in context.Table1
    join t2 in context.Table2
        on t1.Col1 equals t2.Col1
    where t2.Col2 == "xyz"
    select new { t1.Col1, t1.Col2, ... };

You can adapt these accordingly for NOT IN, although I'd recommend never to use NOT IN if you can avoid it - performance will sink and it almost always implies an error in design.

If you absolutely must do it the "IN" way, I suggest going over the answers in this question.