after you install half life, you must install steam and run half life once to make sure that steam can find the files, then restart steam and then install esf, it will automaticly seek for the correct steam folder, and you just have to press "Install" it will then install the game. After that, just open op steam, press "Play Games" search for "Third Party Games" and in that list you will see "Earth Special Forces" click it and you can play it
(Btw, make sure you first download 1.2 and install, then download 1.2.1 and install it, without 1.2.1 you cant play since most servers use 1.2.1 and not 1.2)
**Edit**
Please dont double post, use the edit button (
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To answer your second question, i do not know if u can play it with frontline...but i dont think so....