I think it depends on what you want to explain about some subject. I will show you some example. Let a subject S be 'kodomo', children. and let's take a look following 2 sentences. 1) children is adorable. (explain about subject itself, its characteristic, feature, properties, etc. ) 2) children is running on the play ground. (explain about subject's acting, what subject do) Hence, when you want to explain the characteristic or feature of some subjects, you can use は(wa), like 1). Or you want to explain about the action of subject or what subject do, you can use が(ga), like 2) Now, In japanese. 1) こども(kodomo)は(wa)かわいい(kawaii: adorable) 2) こども(kodomo)が(ga)うんどうじょう(undo-zyo-: play ground)を(o)はしっている(hasitteiru: is running) Note: It's about case in general. There are other many cases that 'wa(or ga)' has different meaning from above.