快速答案
esta 无重音，是阴性指示词；está 带重音，是 estar 的动词形式。
可靠的方法是先判断这个词在句子里的功能，再翻译。
每个词的作用
esta
this, feminine
Use esta for an unaccented demonstrative accompanying or replacing a feminine noun.
está
is, present state
Use está for the third-person present form of estar.
如何选择
- esta: Choose esta if the sentence points to an unaccented demonstrative accompanying or replacing a feminine noun.
- está: Choose está if the sentence points to the third-person present form of estar.
例句
Esta cadeira está livre.
This chair is free.
A cadeira está livre.
The chair is free.
常见误区
They can appear together: esta sala está vazia. The first selects the room; the second states its condition.
放到语境中练习
从你的真实生活中选一个句子，写两遍：一次用 esta，一次用 está。如果两个版本都可以，说明意思怎样变化。