Learn Arabic Vocabulary first.
Sarf before Nahw (grammar) as Nahw teaches how to form a sentence, but Sarf teaches what words to use before forming the sentences.
“Learn Arabic as you learn the [Islamic] obligations and practices.” [Ali b. Abi Tālib]
Shaykh Muhammed bin Saleh Al Uthaymeen (rahimahullah) said:
From the benefits of learning the Arabic language is correction of the tongue with the Arabic tongue which the speech of Allah was revealed in. Because of this, understanding the Arabic language is extremely important. But the sciences of the Arabic language are difficult in the beginning and become easy thereafter. It is the example of a house made of cane (the likes of sugar cane), but its door is made of steel. Meaning, it is difficult for one to enter, but once one does, is then made easy. Due to this, I encourage the student to learn the foundations of the language in order to make the rest easy for him/her.
Understanding Arabic is the way to understand the words of Islâm
According to another hadeeth narrated from ’Umar radiyallaahu ’anhu, he said: “Learn Arabic for it is part of your Religion, and learn how the estate of the deceased should be divided (faraa‘id) for these are part of your Religion.”
This command of ’Umar, to learn Arabic and the Sharee’ah combines the things that are needed, for Religion involves understanding words and actions. Understanding Arabic is the way to understand the words of Islâm, and understanding the Sunnah is the way to understand the actions of Islâm…” [Iqtidaa‘us-Siraatil-Mustaqeem (2/207)]
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“Those who are studying Arabic with goals of understanding the processes and cateories of Arabic language structure will find that descriptions of the morpholoical structure are helpful not only in understanding the theoretical framework for higher levels of achievement and proficiency. Moreover, without a sound grasp of Arabic morphological principles, learners will be unable to make use of Arabic dictionaries.” A reference Grammer Modern Standard Arabic p.45