I have a question with this sentence:
Quali espressioni usano Mario e Gianni per esprimere incertezza e dubbio?
Does it sound also natural if I say "Quali espressioni Mario e Gianni usano ..."?
If so, is there any difference? Thank you!
The second version sounds quite unnatural. It's understandable, but verges on the ungrammatical. Even if you want to stress that it's Mario and Giovanni that are the focus of your sentence, you'd say something like:
Mario e Giovanni quali espressioni usano?
which isn't the finest of constructions, but is very acceptable colloquially to stress that you are specifically interested in what Mario and Giovanni do (as in “I know about Luigi and Antonio but, as for Mario and Giovanni, ...”).
It's a subject-verb inversion needed to avoid any emphatic meaning. If you do not use such inversion it sounds very much emphatically:
"quali espressioni Mario e Giovanni usano?" would mean: we know what expressions they use, but are those really e.g. vulgar, or mischievous, or damaging, ...? Come on!
It sounds just slightly less natural, even if perfectly correct. It depends on what concept you want to stress.
Quali espressioni usano [...Mario e Gianni]
is natural (and easy) because the verb is near to the object, and just after the verb comes the subject, while
Quali espressioni Mario e Gianni usano...
requires you to "memorize" "quali espressioni", then "Mario e Gianni", while you don't know yet what to do with those. But the phrase is anyway correct.
This second phrase has a slight accent on "Quali espressioni", kind of underlining it against "Mario e Gianni", whilst the first phrase is more plain.
=== UPDATE ===
There are cases good for the second form; for example this phrase:
Quali persone guardano Mario e Gianni?
is ambiguous, probably means "which are the persons that are looking at Mario and Gianni?". The second form is not ambiguous:
Quali persone Mario e Gianni guardano?
(what persons are Mario and Gianni looking at?).