how to get subtitles on ipod vid

[Nadream]

Hi guys, i'm struggling to watch naruto episodes with subtitles on my ipod touch, i've downloaded some subbed episodes then converted them to mp4 and synced them to my ipod. But the subs don't come up i've tried going to settings in my ipod and turn captions on but there's no difference. Please can someone tell me how to get the subtitles on my ipod, i'd be really grateful if you could do it step by step. Thanks