Enjoy the drills!
Listen to sentence NO 5 aboveraees mikhad bahat
Listen to sentence NO 6 abovedustam mikhan bahat
Listen to sentence NO 5 abovedanesh amuza mikhan
In the above sentences, since the word نو in Persian ends with /o/ sound, we cannot pronounce /ra/ as /o/ here. Two /o/ sounds close to each other seem odd in pronunciation. That is why we use رو here.
Please note that it is very challenging to issue a general rule for all these things. Besides, I don’t like rules!!
The best way is to use sentences and phrases to get to know how we should use words in our sentences (Just like young children when they learn their mother tongue. They have no idea about the rules but they speak their mother language better than those who are graduated in that particular language!)
Listen to sentence NO 7 aboveirunia futbalo beh basket
As I told you, when we use sentences, we come to know the rules better.
So, try to focus on the Persian sentences in both written and oral form to find out the changes.
Now, you can do it yourself!