～てくる To come to, To become, To continue, To be starting to, Has been ~ing

てくる and ていく combine the て-form of a verb with 来る or 行く to express movement or change over time. てくる suggests movement toward the speaker or a change that started in the past and leads up to now. ていく implies movement away from the speaker or a change that begins now and continues into the future.

ばかりだ attaches to the dictionary (non-past) form of a verb and is used to show that an action, often a negative one, keeps happening or is the only thing happening. In contrast, てくる combines the て-form of a verb with くる to describe an action that began in the past and has continued to the present. Unlike ばかりだ, てくる can be used for both positive and negative situations.

Both つづける and てくる can show the progression of an action over time. つづける attaches to the verb stem and focuses on the continuation of the action itself. てくる uses the て-form and emphasizes that the action started in the past and has continued up to the present.

The verb くる commonly expresses movement toward the speaker, similar to the English 'come.' When combined with the て form of a verb, it can be used literally (e.g., 持ってくる = 'bring') or figuratively to express progression or change over time, like the English 'come to be' (e.g., 雨が降ってくる = 'it will come to rain').

Both ～になる／～くなる and てくる describe changes in state but differ in usage. ～になる／～くなる are used with nouns and adjectives to show something becoming a certain condition. てくる, used with verbs, suggests a gradual change over time. They can also be combined. For example, 面白くなってきた means 'It gradually became interesting.'