I am learning English .I shall see you tomorrow .I can under

I am learning English .I shall see you tomorrow .I can understand you .
I am learning English .
I shall see you tomorrow .
I can understand you .
I must write a letter .
I may come next week .
I did a lot of work yesterday .
I played football yesterday .
①I bought a new coat last week .为什么有的单词变了、哟due不变呢?说说为什么吧例如①还有②
②I have had a letter from Tom .