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.