"She looks pretty" doesn't exist in Japanese.

Today, I’ll be comparing “–sou” and “look…”

For example, “Kono ryouri wa oishisou” is usually translated into English as “This dish looks delicious.”

However, I actually think there are some differences between “–sou” and “look...”. There are certainly times when they’re identical, but that’s not always the case.

This is the definition of “–sou”:

–sou: expresses the speaker's conjecture based on visual information. It concerns an event which might take place in the future or the present state of someone or something (*1)

In other words, it’s used when looking at the condition or appearance of something and discerning from its nature what is likely to happen. 

Example 1: Kono ryouri wa oishisou: Looking at food before eating it and thinking, “If I eat this, it will probably be delicious.”

Example 2: Kono hon wa muzukashisou: Seeing a lot of kanji in a book and thinking, “This book might be difficult to read.”

Example 3: Soto wa samusou: Waking up in the morning, opening the window, seeing snow falling, and surmising, “It’s probably cold outside.”

On the other hand, I think “looks + adjective” is used when judging how something looks. 

look: appear, give the impression of being or doing something (*2)

So, though saying “These looks pretty” when looking at beautiful flowers or “It looks big” when looking at a large house can work in English, translating these as “kireisou” or “ookisou” would be incorrect. 

If you look at a flower and think it’s pretty, in Japanese you would just say “Kono hana wa kirei desu.” The idea of looking at something pretty and thinking “That’s probably a beautiful flower” doesn’t exist in Japanese. Along these same lines, if there’s something large in front of you, there’s no need to guess or consider any possibilities, so you would simply say, “This is big.”

Thus, “She looks cute” is “kanojo wa kawaii desu.” If you see a cute person, you can say “kawaii desu.” However, please be very careful to avoid saying “kanojo wa kawaisou da ne.” The adjective “kawaisou na” is an unrelated word that means “pitiful”, so “kanojo wa kawaisou da ne” means “I feel pity for her.”

I once heard an interesting story about this sort of thing from an American acquaintance. One day, her son fell down at a park and hurt himself, and as he was crying loudly, a Japanese passerby saw him and reportedly said, “Kawaii!” My friend told me that, when she heard that, she thought, “Even at times like this, Japanese people still think foreign kids are cute?”

When she told me this, I felt that it was a misunderstanding on her part. I think the passerby saw the injured child and actually said “kawaisou.” But she probably heard “kawai–” and thought it was “kawaii.” Pretty funny, isn’t it? 

*1 "A dictionary of Basic Japanese Grammar", Seiichi Makino and Michio Tsutsui, The Japan Times

*2 Oxford Advanced Learner's Dictionary", Oxford University Press UK