一开始 + Clause,但/后来 + Clause
The adverbial phrase 一开始 (yī kāishǐ) means 'at first' or 'in the beginning.' It is placed at the start of a clause to set up a contrast with what happens later. It often pairs with transitional words like 但, 后来, or 却 to show a shift in the situation.
一开始我不太习惯,后来就好了。
yī kāishǐ wǒ bú tài xíguàn, hòulái jiù hǎo le.
At first I wasn't used to it, but later it was fine.
一开始大家都反对,但最后还是同意了。
yī kāishǐ dàjiā dōu fǎnduì, dàn zuìhòu háishì tóngyì le.
At first everyone was opposed, but in the end they still agreed.
她一开始学得很慢,后来进步特别快。
tā yī kāishǐ xué de hěn màn, hòulái jìnbù tèbié kuài.
At first she learned very slowly, but later she made especially fast progress.