Is it HR or an HR, little bit confusing!!!

The confusion arises because 'HR' stands for 'Human Resources' but is actually pronounced 'aitch-ah' (or 'aitch-are', if you're American and pronounce the 'r' in words like 'are'), so it begins with a vowel and therefore the correct indefinite article for it is 'an'. When 'Human Resources' is pronounced in full, it begins with a consonant, so you would use 'a'. "An HR problem" and "A Human Resources problem" would be the correct way to write each variant. As far as your email is concerned, the following forms would be usual: "The HR department", "an HR professional", "the HR staff", "the Head of HR", "an HR manager" and so on. You can replace 'HR' with 'Human Resources' in full, if you wish, but if it is clear what you are talking about, it is quite normal - especially in speech and in an email or other informal documents - to use the abbreviation.

Since H has e sound hence an is used. While human does not have e sound thus a is used.

I would only say I wrote to HR there is no reason to use a (not an in the case of Human Resources of course).

HR Manager.

Sorry my friend, I am poor in English Grammar

I think I'll used " the HR" If I talk straight about him / her, but if not maybe Ill use " An HR" But dont count on me, Im not good at grammar

I used to write to HR Management

I think all are grammatically correct but depends on the sentence. The wrong one is "A HR" and/or "AN HUMAN RESOURCES". But better use the complete WORD (in formal letters) not the short letters (acronyms/abbreviations).