Choosing how to express an intention or plan

Start with the status of the plan

These are tendencies rather than rigid stages: real plans can often be described from more than one angle.

One action at three stages

A speaker is considering moving to the same town, but presents the decision from a different point in its development.

Takeaway: The difference is not simply weak, medium, and strong. The first highlights a present decision, the second its continuation over time, and the third the speaker's settled intention.

Personal resolve or concrete arrangement?

つもりだ looks inward at what a person intends. 予よ定ていだ looks outward at the resulting plan or schedule. A personal plan may allow either form, but timetables and organizational events naturally favor the schedule form.

Be clear about whose mind you can report

A direct intention statement most naturally gives the speaker's own intention. When the person is someone else, avoid sounding as though you can simply see their private thoughts. Add a report such as “she said,” quote the person, or make another source of knowledge clear. This matters especially with the forms built on 思おもう, but it is also a useful habit with つもりだ.

Everyday negative plans and solemn resolve

For an ordinary decision not to act, the plain negative form before つもりだ is usually the safest choice. A negative scheduled plan can similarly use the plain negative form before 予よ定ていだ. By contrast, まい builds the negative meaning into a solemn resolution and does not take an ordinary negative verb first.