ようと思う|ようとおもう

1. 1States the speaker's own intention or plan to do something ("I think I'll…").Ex. 1
2. 2In the form と思っている, describes an intention held for some time, and can also describe a third person's intention.

Very common in everyday conversation for announcing plans or intentions, especially ones still being formed. と思う suits a decision made on the spot, while と思っている suggests an intention held for a while. Compared with つもり, it sounds softer and less fixed.

• つもりN5「つもり」attaches to the dictionary form of a verb and is close to 「ようと思う」in the sense of 'intend to', so they are often interchangeable, but 「つもり」sounds more definite and resolute. 「つもり」can also attach to the た form or 「noun + の」to mean 'under the impression that (falsely)', a usage 「ようと思う」does not have.
• ことにするN4When expressing a decision, 「ことにする」attaches to the dictionary form or the ない form and is more definite than the intention expressed by 「ようと思う」, and it can directly describe someone else's decision. It also has the meaning 'treat it as if...', which 「ようと思う」lacks.
• 予定だN4「予定だ」indicates a schedule that has already been arranged; it is relatively objective and certain, usually with a specific time, and attaches to the dictionary form, the ない form, or 「noun + の」. For a mere intention in your mind, use 「ようと思う」— do not say 「予定」.
• ようとするN3「ようとする」means to try or make an effort to carry out an action (often unsuccessfully), or 'to be just about to...', and can also describe natural phenomena; 「ようと思う」only expresses an intention in the mind. Both attach to the volitional form, but they are not interchangeable.
• つもりでN3「つもりで」does not end the sentence; it means doing the following action while holding a certain intention, and can also mean 'in place of / as if...', attaching to the た form, 「noun + の」, and so on. 「ようと思う」attaches only to the volitional form of volitional verbs and directly states the intention, so the two cannot be interchanged.

Compared with ようと思う, つもり expresses a firmer, already-settled intention, ことにする marks a decision that has been made, and 予定だ presents a plan as an arrangement or schedule, which may still be tentative, rather than as the speaker's own thought. ようとする means attempting or being about to do something, while つもりで means "with the intention of" or "acting as if" when describing how an action is done.