إذ تحتاج شياً فتعرفني : is incorrect in meaning. It's better to say : أخبرني اذا احتجت شيئا/ أخبرني عندما تحتاج أمرا ما/ أخبرني اذا احتجت أي شيء / أعلمني في حالة احتياجك لأي شيء Those sentences are all good for the same meaning. You can use all of them with that message..

the word اذ ا< EZZA > is used more commonly in formal Arabic, a better informal replacement which you can use with your friends is لو <LAO>. both words mean '' if'. - the word EZZA can not be followed by a verb in the present, only in the past tense. examples: اذا تحتاجني كلمني>>> incorrect. اذا احتاجتني كلمني>>> correct. :{ meaning : if you need me reach me.} strangely enough, this structure expresses a conditional in the future or the present only but not the past. to express a conditional in the past the word EZZA must be followed by the verb كان KAN. the word LAO makes a correct sentence with past/present tense: لو تحتاجني كلمني<<<correct, but more formal. لو احتجتني كلمني>>> correct, more informal. { both means: if you need me now or in the future}. and as with EZZ, you need the verb KAN to express a conditional in the past. now time to show why the verb فتعرفني is wrong: the verb goes down to the root عرفARIFA meaning knew,learned,acquired information. note that the ''infinitive' of arabic verbs is the form of the verb in the past not the present as it is in english.. the mistake here is the letter ت T. this letter makes the verb in the present but you need a verb in the imperative form. the imperative of the verb is عرف ARRAF. example: اذا احتجت شيئا فعرفني also droping the letter ف F is also correct and more common in informal language. اذا احتجت شيئا عرفني fluency tips: the word عرفني in the above structure is rarely used in formal language: to sound more native use>> اخبرني {formal} or اعلمني {formal}. or OLLI اولي {informal} replace شيئا with حاجه HAGGA.{ informal}. to sum up: a perfect informal sentence should look something like this>> لو احتجت حاجه اولي a perfect formal sentence>> اذا احتجت شيئا فاعلمني

(إذا احتجت أي شيء دعني أعرف ( بذلك

إذا احتجت شيئا فأعلمني إذا كنت تريد اي شيئا، فأخبرني بذلك