Being able to identify root for any word is very important, since by knowing root, you can easily study mature dictionaries, you can study basic meaning shades of the root, study related words from the same root, and then understand the word being studied even better.
So a word has 2 parts, a Root and a Pattern.
Briefly both will be explained inshaALLAH and guide for different levels can be provided at the end. You can't know root without pattern and you can't know pattern without root, and both verify each other. And with experience and effort the skill improves.
After experience and regular study, users can start learning the patterns and by knowing patterns roots can be easily identified and by identifying root in a word, pattern can be easily identified. Then to verify whether the pattern is correct or some other pattern is there, root can be studied further and meaning shade, should be linked to basic root meaning in some way, otherwise another pattern may be applicable.
For example lets start with words every one knows :
مُسْلِم
سَلِيم
سَالِم
Roots of most words are of 3 letters, in all the above letters, the 3 common letters are س ل م
To get pattern for all the above words, simply replace 1st root letter with ف and 2nd root letter with ع and 3rd root letter with ل
مُسْلِم مُفْعِل
سَلِيم فَعِيل
سَالِم فَاعِل
Now in reverse, فَاعِل is a common pattern, using which root of سَالِم can be identified as س ل م
Augmentative Letters :
In a word not all letters, are root letters. The letters in a word which are not root letters are called augmentative letters. Usually, the following come as augmentative letters.
أ ا ت س ل م ن ه و ي
All letters can come as root letters but the above also come as augmentative letters. So this is another indicator.
Practice:
1. Learn the Arabic alphabets in sequence, see 1st post.
2. Visit following tools, beginners and intermediate visit: http://ejtaal.net/aa and advanced visit http://baheth.info
3. Using above start studying words and their roots along with words of the root.
4. If beginners can't identify root, as a 1st step they can study some basic word based dictionaries in which words are arranged alphabetically instead of arrangement according to root. The better method to study words in Arabic is studying according to roots and not according to alphabetical arrangement of words.
5. 1 way for beginners may be to focus on Qur'anic words first, and for that they can note down Surah number and Ayat number in which a word comes. They can then visit word by word Qur'an site and then select the Surah number and Ayat number in which the word is found. Then they can click the word they are studying and this will take to a page where the root for the word will be given. They can also click Qur'an Dictionary on this page to study the words from that root used in the Qur'an.