Putting sou da/desu on the end of a word can represent either hearsay or similarity. Which of the two it means depends on seemingly subtle and arbitrary grammar rules.
But actually that confusing “list of rules” boils down to one simple secret.
Every grammar explanation I have seen makes it seem that there is a complex set of rules that just happen to be what they are and all you can do is learn them by brute force.
But that isn’t really true. Like much of Japanese, the rules make perfect logical and intuitive sense once you understand them.
Tae Kim’s excellent site gives a run-down of all the rules. I won’t try to explain them here. Tae Kim, or your textbook, is much better at that sort of thing than I am. I am just a doll.
What I am going to do is tell you why those rules actually make sense and are not just an abstract set of rules to be learned.
For example, Tae Kim tells us that for the “seems like” meaning:
- Verbs must be changed to the stem.
- The 「い」 in i-adjectives must be dropped except for 「いい」.
- 「いい」 must first be conjugated to 「よさ」.
- For all negatives, the 「い」 must be replaced with 「さ」.
- This grammar does not work with plain nouns.
One might also add that na-adjectives have the sou attached directly to them (rather than having putting da/desu between the adjective and the sou as you do when you mean “I heard that…”)
But the truth is that once you know what you are actually doing when you are doing all this, it all becomes very easy and intuitive, and there are no “rules” to remember.
“The Simple Secret of Sou” is chapter 8 of Unlocking Japanese. You do need to have read the previous chapters to grasp what it is saying, but the whole book is very straightforward. You can read it in an evening and Japanese will become simpler, and more intuitive for the rest of your life.
The simple secret of sou is part of a ground-breaking new way of seeing Japanese that simplifies not just sou, but everything! If you are serious about learning Japanese you owe it to yourself to learn the secrets that make it easy and intuitive.