I think most words we think of as synonyms in English are not exactly the same. If you're a native speaker, you'll probably just use them instinctively. The difference may be very subtle, and difficult to explain, but "feel" right or wrong. For example, you'd never normally say that somebody was "usual". That word would be more for habits and routines than just to describe what somebody is like. "Normal", "ordinary", (and probably "regular" in US English) are so similar, but you could use them to describe the person in a way you couldn't with "usual", but even they aren't the same as each other. If you said someone was normal, it would be neutral, but if you said they were ordinary, it could have the meaning that they were nothing special. The differences between the words are like different spices. Common and simple are further away in meaning from those other ones. Simple could mean uncomplicated or it could mean that you are not mentally quick. Common would be more normal with animals and things than people as the opposite of rare. For a person, it sounds like you are looking down on them and thinking that they behave like a pig :-)
Not all languages, and not all the time. But it is common in the Romance languages for the syntax to be mostly noun-adjective (for all noun phrases, not uniquely in the direct object).
There really is no "why". Different languages develop differently and have different rules. Italian speakers and Spanish speakers are working to get used to the English way of saying adjective-noun.
Interesting question (noun-adj or adj-noun?) and hard to get a feel for. I read in another discussion of the fitum a very useful tip: BAGS for the adj-noun form: Beauty (bello/a, brutto), Age (giovane, vecchio), Goodness (buona, cattiva), Size (oiccolo, grande). Seems to cover 80-85% of the cases. I would like to give credit to the contributor, but forgot her name (pretty sure it was a 'she').
BANGS: beauty, age, number, goodness, size. Although as I learned while studying Spanish (which possibly also applies to some degree in other Romance langages), sometimes whether it comes before or after changes the meaning.