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…”
What I am going to tell you is what you are actually doing when you are doing all this, and how knowing that makes it all very easy and intuitive.
Essentially, for the “seems like” meaning, you are grafting sou onto the word so that it becomes a new adjective. For the “I heard that” meaning you are completing the statement and then adding sou to mean “so I heard”.
Sou meaning “seems like”
For the “seems like” meaning you are simply changing a verb or an adjective to become a new adjective.
- oishii (i-adj: delicious*) becomes oishisou (delicious-looking)
- manzoku (na-adj: contentment/contented) becomes manzokusou (contented-looking)
- ochiru (vb: fall) becomes ochisou (fall-looking: ie, looks as if it’s about to fall)
This is the reason you use the “seems like” sou in this way. Each time, you are creating a new na-adjective ending in sou, which means [original word]-looking or -seeming. Remember this and the rest makes sense.
Sou Meaning “I heard”
With the sou that means “I heard” you are not grafting sou onto what comes before it. You are completing the statement and then adding the rider “so I heard”.
oishii sou desu = It is delicious, so I hear
Note that this time we are not grafting sou onto oishii by removing the last i and replacing it with sou. We are completing the statement oishii (=it is delicious – remember that i-adjectives contain the “it is”/da/desu within themselves) and then adding the rider that this is what you have heard.
kirei da sou desu = It is pretty, so I have heard
Na adjectives do not contain da/desu within themselves, so we add da to the adjective to make it a complete statement before adding the rider sou desu.
(Note: I would recommend reading “I and Na Adjectives: what the textooks don’t tell you” because a lot of what makes these “rules” seem confusing is not understanding how adjectives really work)
ame ga furu sou desu = It is going to rain, so I have heard
Contrast this with
ame ga furisou desu = It is rainfall-looking (seems as if it will rain)
In one case we graft sou onto furu (using the masu-stem method of connection) making the new adjective furisou (=fall-seeming). In the other case we use the complete verb furu and then add the sou rider.
So now it is easy
Once you know this, the whole list of “annoying rules” becomes simple and obvious. We are doing the same thing in each case, in the standard way that Japanese always does these things.
The only tricky exceptions are:
When you connect sou to ii (good) to make the adjective “good-seeming” it becomes yosasou. This isn’t really confusing as ii always becomes yo when it is conjugated in any way. For example the past tense of ii is yokatta.
The only really exceptional thing is that nai becomes nasa when connected to sou to make a negative “doesn’t seem like” adjective.
Tayorinai (unreliable) becomes tayorinasasou (unreliable-seeming).
So all that list of mind-muddling rules boils down to one that actually doesn’t make instant sense once you know how it all works. And nasasou (unlikely) is a very useful word on its own so you would be learning that anyway.
Cultural notes on sou meaning “seems like”
Words like omoshirosou, (interesting-seeming) are much more used in Japanese than the English equivalents are. The reason for this is that in Japanese we tend not to express an experience directly unless we have had it directly. So if I have done a thing and think it was interesting I say omoshiroi, but if I haven’t done it but think it would be interesting I have to say omoshirosou.
In two cases the –sou adjective of a word doesn’t mean what you might think.
Kawaii means cute, but kawaisou doesn’t mean cute-looking but pitiable. Be careful not to call someone’s baby kawaisou!
Erai means great, magnificent, admirable. However erasou doesn’t mean admirable-seeming. It means important-looking in the sense of trying to look important: arrogant. Again, don’t call anyone erasou because you think she looks distinguished!
These are the only two common cases where the sou-adjective doesn’t mean what you expect.
*Oishii can mean something more than just delicious. Go here to learn the full meaning.