According to the book I am reading, "Sakura is beautiful" is written as
and "Sakura is a beautiful flower" is
,but when using たかい the na form isn't used. For example:
It is because there are い adjectives and な adjectives.
The attributive form of a な adjective ends with な, such as in きれいなはな.
The attributive form of an い adjective ends with い, such as in たかいやま.
When they are placed before です, in the case of an い adjective, you use the plain form, such as in ふじさんはたかいです. In the case of a な adjective, you use the stem of the word, such as in さくらはきれいです.
This is unfortunately one of the parts of Japanese that you'll have to just remember. Some Adjectives can also be said with い AND な。
It happens with a few adjectives, and this occurs in daily speech as well.
In general, especially when you're first learning, I would simply use the い form for all ”い” adjectives. Realize that there are specific な only adjectives as well (which do not take い).
If you want to delve further, this has been answered in depth here: