How do you say "I think X is Y"

Question

I've been thinking of this for a while and I need a bit of a clarification. For example, if I want to say "I think he is a good person", would it be:

a)　私は彼がいい人と思う 　

or

b) 私は「彼はいい人」と思う

Also, if it has already been established from beforehand that I am the topic of the sentence, would it then be possible to shorten this to 彼がいい人と思う or 彼はいい人と思う (whichever one is right from the first part)?

Answer 1

When you use a quotative-と, the "quote" will basically be a normal non-polite sentence. That is, you can safely use は inside the quote, and you should not drop だ. Brackets are usually not used in a simple case like this. Therefore, the correct sentence is:

私は彼はいい人だと思う。

The polite version is:

私は彼はいい人だと思います。

As you may already know, 私は is normally omitted, so the following is almost always more natural unless you have to emphasize "I":

彼はいい人だと思う。 / 彼はいい人だと思います。

Note that this does not mean something like "He thinks (about) a good person". See this question for the reason.

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There are exceptional cases regarding と:

• だ is sometimes dropped in informal sentences (known as だ抜き). 私は彼はいい人と思う is acceptable in a casual conversation.
• A + を + B + と + verb is a different construction from this, and you can safely drop だ in such cases.

These are discussed in this question, but if you are a beginner, you may want to learn about them later.