Question: hello. When do I use ng in a sentence verses ang in a

There is no simple way to explain this. The question itself shows a fundamental misunderstanding of how this works that needs to be resolved. Please read up on markers:
seasite2.niu.edu/beg inning-tagalog-1-gra mmar/tagalog-markers /And for you second question I believe it would actually be:Bakit sila tumatawid SA ilog? You will find out more about that at that page I linked to.

in this case "ng" is used as an object Tumatawid kami NG tulay ...You can also say Tumatawid kami SA tulay ( sa is used as a direction) ..
***Bakit SILA tumatawid ANG Ilog ??? It should be Bakit sila tumatawid ng/sa ilog..You cannot have two topics "sila" and "ang ilog"

Ang is used to mark the topic (known information), while ng is used to mark the comment (information about the topic). It is somewhat similar to how "the" & "a" are used in English (compare "the dog" vs "a dog")For example, the sentence "Kumagat ang aso ng pusa" may be translated as "The dog bit a cat," while "Kinagat ng aso ang pusa" may be translated as "A dog bit the cat."You can read more about the uses of ng and ang here: learningtagalog.com/ grammar/markers/comm on_uses_of_ng_marker s.html learningtagalog.com/ grammar/markers/uses _of_ang_markers.html