"I think that test is probably easy."
「～と思います。」Is a grammar set for "I think~." You always have the と attached to the 思う.
The sound that comes before the と思います will depend on what the previous word is. The following are just the basics, there are many other ways to use grammar combinations before と思う.
If it is a な adjective or a noun use だ.
本だと思う ＝ I think it is a book (noun)
いぬだと思う ＝ I think it is a dog (noun)
かんたんだと思う ＝ I think it is easy (na-adj.)
ひつようだと思う ＝ I think it is necessary (na-adj.)
If it is an い adjective you do not need the だ.
やすいと思う ＝ I think it is cheap
おもしろいと思う ＝ I think it is interesting
If it is a verb, keep it in its dictionary form.
ゆきがふると思う ＝ I think it will snow
来ると思う ＝ I think (He) will come
Past is also okay.
かえったと思う ＝ I think (He) went home
食べたと思う ＝ I think (He) ate