Is there any difference between "e" and "este"?
Is the full word more formal?
Can I just use e forever?
Indeed, as the single e is faster, it is slightly more often used when speaking, especially in casual and informal environments. Sometimes este sounds better, but I can't think of specific examples.
It would be correct to write "sumele" Because we use to say "the sum"?
'Sumele' is the plural for 'the sums', where 'suma' is 'the sum and 'sumă' is 'sum'