I was defeated by you 句子没问题吧？

1. 语法上对，但语感不好。最好不要用defeated by...
可以说：
You defeated me.
或
You beat me.

2.“我要工作到7月份”是指七月底吧？
I'll be working until the end of July.
用将来进行时表达“延续”的意思。

1.
最好不用by you

虽然语法上没问题,感觉很不地道

直接回答

I was defeated

2.
I will keep working till July

当然可以，这里的you是被动句的宾语，改成主动句它就成了主语。
希望对你有帮助

我要工作到七月份——I have to work till July.
这里的“要”有种“不得不”的含义，用"have to"

I will keep on working till July.
keep on doing sth.继续做某事

可以