This is hard one because it moves around from what IS the case to what happened in the past. If she has been seperated for 10 months, we can assume the husband's interaction wtih the kids is now over. But perhaps they are still hoping to work it out. It could go two ways:
I've been married for 16 months and separated for 10 of those months. The reason for the separation is because my husband thinks my kids, from my previous marriage, 13 and 10, will never learn to behave. I had been a single mother for 7 years and worked 2-3 jobs before I met my husband. So, I admit my kids are lacking manners. I believe the problem stems from him not communicating with the kids. He completely ignores them and the only time he speaks to them is when he is correcting them. Of course the kids resent him. This keeps the issues in the present.
I've been married for 16 months and separated for 10 of those months. The reason for the separation is because my husband thought my kids, from my previous marriage, 13 and 10, will never learn to behave. I had been a single mother for 7 years and worked 2-3 jobs before I met my husband. So, I admit my kids are lacking manners. I believe the problem stemmed from him not communicating with the kids. He completely ignored them and the only time he spoke to them was when he was correcting them. Of course the kids resented him. This says is all in the past
In fact, it's probably a mixture of both - they are still trying to solve the problem so some elements are past and some are present.Maybe he still thinks, and they still resent, but his communication with them is past.
(PS - Don't use "the reason... is because.")