Hi. One woman here helped me a lot, when I had similar question. Have a look at it:
' Stems VII to X (let's disregard the rare stems from XI upwards) start with اِ, i.e. alif with a kasra. In these stems, you never write a ء wherever you have a form that starts with "i", i.e. all past tenses, all imperatives and the masdar. It is a very simple rule. Please note that many Arabs err in them, and some will even correct you wrongly'.