つもり

1. 1Intend or plan to do (or not do) something, stating a firm, decided intention.Ex. 1
2. 2In the past form つもりだった, had intended to do something but the plan did not happen.Ex. 2

Neutral register, common in everyday conversation and writing about personal plans, such as travel, study, or future decisions. Compared with ようと思う, it sounds more settled and firmly decided. Unlike 予定だ, it expresses what the speaker personally intends rather than an arranged schedule.

• ようと思うN4ようと思う attaches to the volitional form of a verb and matches only the 'intend to' sense of つもり. The two are often interchangeable, but ようと思う stresses that you are 'thinking of' doing it, so it sounds softer and less firm; to describe a third person's intention, use ようと思っている.
• ことにするN4ことにする centers on the act of deciding, and ことにした means the decision has been made — it can also describe another person's decision — while つもり refers to the speaker's own intention. When attached to the た form, 〜たことにする is a deliberate 'treat it as,' whereas 〜たつもり usually means 'under the impression that.'
• 予定だN4予定だ describes schedules that have already been set — fairly objective plans, usually paired with a specific time — so it does not suit vague personal intentions, and it never carries the 'under the impression that' sense. When asking a superior or elder about their plans, ご予定は is more appropriate than つもり.
• つもりでN3つもりで is つもり with で added, a linking form that keeps both senses — 'intending to' and 'treating as' — but it cannot end a sentence: an action performed with that mindset must follow, as in 冗談のつもりで言った. To state an intention at the end of a sentence, use つもりだ.
• たいN5たい expresses only the wish 'want to do,' not a settled plan, whereas つもり means 'plan to do.' たい attaches to the ます form minus ます (e.g., 行きたい), conjugates like an い-adjective, and has no 'under the impression that' usage.
• ことになるN4つもり expresses one's own intention, while ことになる refers to arrangements decided by someone else, an organization, or circumstances (it can also report your own decision, in a more indirect way), and it can state a calculated result. When attached to the た form, 〜たつもり means 'under the impression that you have done it,' while 〜たことになる means 'it amounts to having done it.'

つもり states a settled personal intention, whereas ようと思う is a softer, more spontaneous intention, たい expresses desire rather than intention, ことにする marks the moment of deciding, ことになる marks a decision made by circumstances or others, and 予定だ refers to a fixed schedule. つもりで adds a clause meaning "with the intention of / as if," describing the attitude in which an action is done.