Without context, how can one know whether this should be "sure" or "safe"? Both should be correct, in my opinion.
Secondo me, certo/certa/certe is more like the English "certain" - eg, being sure of an opinion.
Sicura/sicuro is more like "sure" or "safe".
If you keep 'certo' for 'certain' and 'sicuro' for 'sure', I'm sure you'll be safe. Hmm...
A right italian sentence that you can use to translate "I am safe" is "Sono al sicuro".
I also wrote, "I am safe." Good English grammar, common phrase, sentence can be used in many settings......uh.....???? Am I missing something? If so, can someone explain?
I think it really doesn't matter at all. You might make it "sicura" if you're female, and "sicuro" if you're male, but there's no grammatical difference.
Careful: sono can be 'they' in some cases, but sicura is only for a single lady ;)
so how would we say.... they are sure? Sono... securo (a)... seems singular. What is the plural for more than one ?
You may want to know the difference between cert... and sicur..., rather than ...o and ...a. If so, this has already been answered rather well by RobBlaney above. Duo asks us to read existing posts first, to avoid duplication.
In....what's better : saying sono certo or sono sicura ?......... it's a bit confusing. In certO... is the O ending for I/first person singular... or O ending for a Man saying I am sure?
Same with sono sicurA... Someone above said the A ending is for a Female speaker...
So if it were a Man would we/he say: Sono sicurO?
Which would mean we've got to change endings on those words (and others) depending on our own gender. And we need to do so When? How do we know which words to switch from O to A? Hope this question makes sense, it is a bit convoluted, sorry.
Most adjectives referring to the masculine end in -o; most feminine end in -a. Most masculine plural adjectives end in -i; most feminine in -e. So franwy you are correct that a man would say "sono sicuro". A woman would say "sono sicura"; some men (or a mixed group) would say "siamo sicuri"; some women would say "siamo sicure".