фраза

фраза

(Более естественный вариант, без повторов. More natural option without repetition)

Я пытаюсь писать по одной фразе на русском каждый день, чтобы улучшить свои навыки.

фраза it word is mainly used in everyday speech, a phrase may be any group of words, often carrying a special idiomatic meaning.

In your case a sentence is a textual unit consisting of one or more words that are grammatically linked.

Я пытаюсь писать один одну фразу на русском языке каждый день, чтоюбы улучшать мой русский язык.

Let me suggest some other improvements. Phrase "писать одну фразу каждый день" might be interpreted as "write the same phrase each day" (as the literal translation from Russian - "write one phrase each day" might be perceived in English). To remedy this you can change "одну фразу каждый день" to "по одной фразе каждый день" which would imply that the phrases should be different each day - "one phrase per day", though the precise reverse translation of "one phrase per day" from English would be "по одной фразе в день". You can shorten this to "по фразе в день" - "a phrase per day".

Next, "улучшать мой русский язык" means "improve my Russian language". But I guess your goal was to say "improve my Russian", therefore you should drop the word "язык" ("language")

Phrase "на русском языке" sounds a bit formal and may be changed to "на русском" or "по-русски" for conversation purposes.

Now we got "Я пытаюсь писать по фразе в день на русском, чтобы улучшать мой русский" - "I try writing a phrase per day in Russian to improve my Russian". I would suggest you to change last "Russian" to "skills".

The final result: "Я пытаюсь писать по фразе в день на русском, чтобы улучшать мои навыки".