The easiest way is to leave your sentences separated and put appropriate conjunction such as soshite, demo, sorekara, sorede, sokode etc. before the second sentence. "I went to the store. In the store, I bought groceries. Then, I went to see a movie." わたしは　そのみせにいきました。そこで　たべものを　かいました。それから、えいがをみにいきました。 #Sorry, I didn't quite get what you meant by "check out a movie," so I changed your sentence a little bit.

Plain form is the most basic form where any Japanese learners have to grip with. In fact, it is easier for you to master the rule of deviations from the plain from. Interstingly, accent of te form and ta form are same as the plain form. As for combination of sentence. it depens on the category of meaning that you want to say. For instance, when you ask somebody, you need to use te form.

That question is way too general. There are a variety of ways to do this. You need to change the verb into one of a variety of "connective" forms.