You can use '은', '이' with a consonant placed under a vowel. There are some examples. -꽃은 아름답습니다. (A flower is beautiful.) -말이 달립니다. (A horse is running.(or runs)) You can use '는', '가' if there is no consonant placed under a vowel. -바다는 여기서 멀다. (The sea is far from here.) -나비가 날고 있습니다. (A butterfly is flying.) So you can use them like that.

http://wildkorean.blogspot.com/2011/12/topic-marker-and-subject-marker.html - hope you find it helpful