Usually we use crush, although this is the American way of saying it. Eg: I have a crush on him. (我暗恋他） In UK, they tend to use the word fancy. Eg: he fancies her (他暗恋她）

to be secretly in love

你说的对，是用have a crush on sb。但是只要是对方不知道，所有的喜欢都可以叫“暗恋”是吧？中文无非刻意强调了这种状态，但在英文里我感觉没这么麻烦，所以口语里你还能说sweet on sb（淡淡的爱），be into sb（较强烈的爱），sb turn me on（爱的感觉）等，希望对你有帮助

They simply don't say that as often as the Chinese.

Secret Love