Hello Yukichi.

I hope this helps :)

Today's dairy

Today we had an EPT test (EPT starts with a vowel so you need ''an''). It was a speaking test. It is a very important test . But, I can't speaking English well. (after ''can'' you must use infinitives / there's no ''ing'')
The speaking theme was "About your family and about your country ".  I think it is an essay theme and a simple one*. But, I could not** to make English suntans sentences about it. I have to study English more.

* I think it is an essay theme (a noun) and a simple theme (it also has to be a noun, but in order to avoid repetition you will use ''one'')

I think it is interesting (adjectives) and simple (adjective).    -     You should use the same parts of speech when you use ''and''. 

** After modal verbs, like can, could, must, will, would....you must use infinitives without ''to''. 

I can go...I could go...I will go....I would go (Don't say I can to go, I could to go...NOT CORRECT).

Today's dairy

Today we had an EPT test. It was a speaking test. It is a very important test. But, I can't speaking English (very) well.
The speaking theme was "About your family and about your country " I think it is essay theme and simple the essay's theme was simple and easy. But, I could not to make English suntans sentences about(?) it. I have to study English more.

ｘ　aとanのルールは日本語で説明するのは難しいだがwikipediaで読みできます 。- https://ja.wikipedia.org/wiki/%E8%8B%B1%E8%AA%9E%E3%81%AE%E5%86%A0%E8%A9%9E#a_.E3.81.A8_an_.E3.81.AE.E9.81.95.E3.81.84
ｘ　「very」が必要ではありませんが文に足したらもっと自然そうと思います。
ｘ　「it is essay theme and simple」はそんなに間違いではありませんが「the essay's theme was simple and easy」に変わったらもっと自然そうと思います。
ｘ　ここに「to」はいりません。
ｘ　「suntans」は「（複数の形）日焼け」なのです。「sentence」は「文」なのです。
ｘ　ここに「about」の意味が分かりません。テストで何のことに文を作る試してみたのですか？

この間違いよりおｋです。

Today's dairy

Today we had a EPT test.  -->>    Today we had an EPT test.

It was speaking test.    -->>            It was a  speaking test.   

It is very important test .   -->>      It is  a very important test .

But, I can't speaking English well.  -->>      But, I can't speak English well. 

The speaking theme was "About your family and about your country "

I think it is essay theme and simple. -->>   I think it was a simple, descriptive -style  speaking topic.

But, I could not to make English suntans about it.  -->>   But, I could not to make English sentences  about this topic. 

I have to study English more. 

***Why don't you try to write out what you want to say  about your family and your country and post it  here and  then we can help you  use English  to describe this topic .