田中さんはこの小説を読んだ（そうだ）- do we need そうだ all the time if we did not actuality get the info from the person?

Question

Example 1

Me: 明日は先生が来る（そうだ）。

Example 2

Me: 田中さんはこの小説を読んだ（そうだ）。

Assume both Examples 1 and 2 are the information I heard from other people, not from 田中さん or 先生.

Can I say the Examples without そうだ?

In what situation can I not use そうだ? If the sources of information are trustworthy?

Answer 1

そうだ is a hearsay marker that roughly corresponds to "I heard" or "[They] said (to me)". It's useful when you want to express that it's secondhand information rather than a fact you confirmed yourself. But it's not mandatory at all if you don't want to express such a nuance. The difference is simply:

• 明日は先生が来ます。
  Sensei will come tomorrow.
  _{(You're expressing this as a simple fact.)}
• 明日は先生が来るそうです。
  I heard Sensei will come tomorrow.
  _{(Who you heard this from is not expressed; it may have been from the teacher themself or someone else.)}