How do I write the verb "To be"? 居る or いる？

Question

Do native Japanese speakers write 居る or いる？

Answer 1

When it's used as an ordinary verb ("to be; to exist; there is"), いる is mostly written in hiragana. When it's used as a subsidiary verb ("is ～ing; have ～ed"), いる is almost always written in hiragana. Today, using 居る would look old-fashioned and stiff. Novels 100 years ago tended to use 居る a lot. Unless you are interested in creative writing, you don't have to use 居る at all.

Answer 2

Do native Japanese speakers write 居る or いる？

I know the use of them is very complicated, but it depends on the phrase like:
1. 私は家に居る。あなたは外にいる。
2. 私の家にいる猫がかわいい。
3. 私が家にいる時に電話があった。

The use of them is not so strict but roughly:
the verb 居る is used in its predicative form to describe somebody is indoors, while いる is used to describe that somebody or some thing is outdoors, or it is also used when the verb is used in a modifier.

The kanji 居 originally means to stay at home, so 居る fits to stay indoors.
As for the 猫｛ねこ｝ cat , the いる is used in stead of to be kept as a pet.

And the other reason いる is used in stead of 居る is that 居る looks heavy/thick/dense(?) than いる when written in a line of a phrase, so if it is not misread いる is often used in stead.