In Arabic, as far as my beginners Arabic CDs have so far taken me, there is no difference between the English:
In Arabic, they all correspond to /ɑːnə/. I’m not going to write it in Arabic script because I can’t.
As such, as Arabic speakers grapple with learning English, they are confronted with a constant choice where in Arabic they don’t need to choose.
I currently teach many beginner Arabic speaking students in a multi-lingual classroom. I have presented the three options (above) from the beginning of the course and it seems that students are assimilating them much faster once the options are pointed out explicitly. We chant together in class “I, I’m, I’m a” (with emphasis on the shwa) in class and then select the correct one depending on the target language of our class.
Job? Students choose “I’m a” = /aɪmə/. You can present the IPA to reinforce it as a language chunk.
Present simple verbs? Students choose number 1 “I” = /aɪ/
Adjectives or present continuous? Students choose I’m
The same technique can work with different subject pronouns.
You He She
You’re He’s She’s
You’re a He’s a She’s a
I think in the past we have been tempted to gradually introduce these items so that students don’t get overwhelmed. I see the usefulness of this opinion. However, I suggest that students react better to having a choice rather than guessing about options that are hidden from them by coursebooks (the old “not until Unit 5!” chestnut).