What to say after the plain form, part 1

Here's a recap of the idiomatic expressions to use after the plain form of a verb. I'm splitting it up because it's a lot to explain.

First, just to be clear on what the plain form is: it is the form of a verb that, when used at the end of a sentence, is the less polite form of the -ます forms we've been learning. You also use the plain form in polite communication, but then only in certain contexts, such as the idioms discussed during class and in this post.

We've learned the present affirmative, present negative, past affirmative, and past negative of the polite form, which are respectively (for 'to eat'): たべます,　たべません, たべました and たべませんでした.
The plain form equivalent of this sequence is: たべる (this one is known as the dictionary form), たべない, たべた, and たべなかった.
The -te form (for example, たべて), is neither polite nor plain, and so only exists in one form.
For a full list of all (common) conjugations of a verb, visit the Ultra Handy Japanese Verb Conjugator.

Anyway, on to the first idioms. They are as follows:
～とおもいます (in kanji: ～と思います).　This means 'I think that ~'. For example:　にほんごを　はなすと　おもいます。 I think (s)he speaks Japanese.
～といいます. This means 'X says that ~'. For example:　がくせいは　レッスンを　わすれると　いいました。　The student said he forgot the lesson. Or: The student said, 'I forgot the lesson'. Note that I changed the present tense いいます into the past tense いいました.
～どう　です. This means 'I have heard that ~'.
～か　どうか. This means 'whether or not ~'. Sensei gave us this example: アントニさんは　きょう　くるか　どうか　わかりません。 'I don't know whether or not Antony will come today.'
～でしょう. This expresses conjecture (that is, not being sure of something) and roughly translates to 'probably will'. Literally it's the volitional form ('let's X') of です, to be. On the Web, I found this example: ちちは　みせで　バナナを　かう　でしょう。 Dad will probably buy bananas at the store.
～かも　しれません (in kanji ～かも　知れません). This is much weaker conjecture, translating to 'may' or 'might'. Literally it just means 'I don't know if ~'. Sensei's example was: アントニさんは　きょう　くるかも　しれません。 'Antony may come today' (literally 'I don't know if Antony will come today').

One thing I can't figure out is how you figure out the scope. Let me explain. Take the sentence ちちは　すしを　たべると　おもいます。 The parts of the sentence mean: dad(topic) sushi(object) to-eat(sentence-as-object) think.
Now the way I see it, this sentence can mean two things:
1. 'I think dad eats sushi.' In this case, ちちは is part of the relative clause ちちは　すしを　たべる, 'that dad eats sushi'.
2. 'Dad thinks he/she/etc eats sushi'. In this case, ちちは would be part of the main clause と　おもいます, 'Dad thinks that X'.　I'm not sure if that is a valid reading of the sentence, or if there's some way to tell the two apart. I'll try to find this out.