Resolved questions
چتور همان نیستند؟ چه کلمه مشترک ترین است؟

1.
چتور همان نیستند؟ چه کلمه مشترک ترین است؟
چطور همسان نیستند؟ چه کلمه‌ای معمول‌تر است؟
"Common" has two different meaning in Persian.
a. common = usual (occurring, found, or done often; prevalent) = معمول
b. common = shared by, coming from, or done by two or more people, groups, or things = مشترک
In this question, we should use معمول

2.
فرد ~ individual = a single human being as distinct from a group
It is rarely used in informal conversations.
منفرداً ~ individually

شخص ~ person
It's used in both informal and formal conversations.
شخصیت ~ personality

آدم ~ human - man - guy
It's used in informal conversations frequently.
He's a good guy ~ آدم خوبی است.

3.
صحیح = / = موردعلاقه
These two words have two different meanings.
صحیح ~ correct , right
موردعلاقه ~ favorite , beloved


4.
تفریح ~ سرگرمی
These two words are the same as a noun and used frequently in speeches.
But we use تفریح in verbs : تفریح کردن
سرگرمی کردن is incorrect and we don't have a verb like this.

5.
ناز ~ با مزه ~ با نمک
These three words are the same almost everywhere and used frequently in speeches.

6.
اول = نخست = first
These two words are the same but اول is formal and informal and نخست is just formal.

اولین = نخستین = the first
These two words are the same but اولین is formal and informal and نخستین is just formal.
These words are superlative adjectives.

اول - دوم - سوم - چهارم - پنجم - … ~ first - second - third - fourth - …
اولین - دومین - سومین - چهارمین - پنجمین - … ~ The first, The second, The third, The fourth , The fifth - …