上手な文章を書くためには、文末を、「です、ます、ました、でした」とするか、
「だ、である、であった、あるいは動詞の原型」など、に統一をして書くということが、日本語の決まりです。

たとえば、行きます。行きました。と、行く、行った、などにあたります。

それらが混じると、良い文章ではありません。（特に意図がなければ統一します。）

通常は、「です、ます」が丁寧です。お手紙などは、「です、ます」を使うのがよいでしょう。

しかし、論文、などを描く場合は、「だ、である、動詞の原型」が

よいでしょう。なお、「だ、である」文章はやや言い方がきつく感じられたり、

断定的な言い方になりやすいため、通常は文末を

「であるようだ」「のように思われる」「とされている」など和らげる（柔らかくする）ことがあります。

なお、話し言葉が混じる場合は、色々な文章が混じります。例えば、

なみちゃんと、さわちゃんが話をしています。

「お前なんて嫌いだ」

そういわれてなみちゃんは困りました。

等では、会話と、そのほかの文の文末はそろいません。

です、ます、あるいは、だ、だった、などを一つの文章でそろえると、

自然ですし、また、文章自体も、とてもきれいに響きます。

When I started learning English at first, I could learn from everything, but after learning some, according to the purpose how I want to use English, I understand I can choose effectively how much I need to and what I don’t need to learn.<o:p></o:p>

Let me explain, if your future dream is you tuber speaker in another language, and you want to become a very trustworthy news source on it, you need to learn from native because to check what you are trying to say is correct or not. If what you are talking have so many mistakes which only natives can tell, they will not keep listening even though the content you create is great.
When you think you are talking interesting information, you don’t have to be perfect. These people need to talk with passion or confidence, and to get these, many chances to talk others maybe helpful for them to make their dreams come true.<o:p></o:p>

I head two you tubers recently, they talk in Japanese.<o:p></o:p>

One is from England, and his Japanese is not correct, but still the content is great he gathers many Japanese people. That is the case you can succeed even your language is not perfect, so it may be good for you to start if you have a good resource which you can share, you may not have to wait until your learning language becomes perfect.<o:p></o:p>

I also say you need to value your language ability though,because his balance is good and if you commit so many mistakes more than him, we can’t understand. What he talks is great theories about music, and make some mistakes, but still we can understand. Try to talk people and be humble, they will tell how much they can understand.<o:p></o:p>

The other you tuber, is a Korean girl who have learned Japanese in Korea! Her Japanese grammar is perfect, but Japanese can tell she is from Korea because of her accent.<o:p></o:p>

I discovered through her you tube program, that even you have some accent, and<o:p></o:p>

the grammar is perfect, we admit that language is perfect.
Only when the accent is so strong, no one will listen to. Takanori Jinnai is a great comedian, he had succeed some great show in New York I believe, but at the time he didn’t speak a lot. He tried to succeed in You tube in English also, but this time he needed to speak a lot and  his accent was too strong for native English speakers so not so many people liked it.<o:p></o:p>

Any time you can start doing your dream and be humble to learn ,hear how much you can express yourself and think what you need to improve or don’t have to take time to learn so much,

If you have a dream to use another language, try to concreate your dream through starting to do something and have some experiences, so you can tell how much you will learn and what kind of words you need, what point you need to improve.<o:p></o:p>