Here's my explanation:
هي تذهب: The word " تذهب" is a verb usually represent the meaning of present simple in English.
She goes to school everyday= هي تذهب إلي المدرسة كل يوم
عنما تذهب إلي هناك تشعر بالحزن= When she goes there, she feels sad.
2. هي ذاهبة : The word " ذاهبة" is "اسم فاعل" or sth like an adjective in English, It can represent too many tenses according to the context.
هي ذاهبة إلي المدرسة الآن = She is going to school now -> present continuous
رأت كلبا وهي ذاهبة إلي المدرسة = She saw a dog when she was going to school -> past continuous
اتركها تنام فهي ذاهبة الي العمل غدا باكراً = Let her sleep, She is going work early tomorrow. -> future
It can also represent your intention or plans
أنا ذاهبة للمطعم
Hope that helps ^_^