In general, 「も(mo)」 is used mainly for emphasis. Thus, it is not necessary to keep writing も in most of the sentences. On the other hand, unless there exists particular temporal adverbial (such as tomorrow, etc.), it is better to express the sentences in simple present tense. 1. I (will) write a letter to my friend. [わたしは　ともだちに　てがみを　かきます] 私は友達に手紙を書きます。 Examples for usage of「も」: 私も友達に手紙を書きます。-- This sentence works, if it has been fore-mentioned that, someone else (e.g. friend's parents) writes a letter to a friend, and the speaker (I) also writes a letter to a friend. 私は友達にも手紙を書きます。-- This sentence works, if it has been fore-mentioned that, the speaker (I) writes a letter to someone else (e.g., speaker's parents), and then also to a friend. 私は友達に手紙も書きます。-- This sentence works, if it has been fore-mentioned that, the speaker (I) writes something else (e.g., postcard) to a friend, and also a letter. それに私は友達に手紙を書きます。-- This sentence works, if it has been fore-mentioned that, the speaker (I) does something (e.g., call the friend by phone), and also writes a letter. Hence, where to put も for emphasis is very important. 2. I wrote a letter to a friend. [わたしは　ともだちに　てがみを　かきました] 私は友達に手紙を書きました。 3. Mr. Furukawa (will buy) buys a newspaper at the train station. [ふるかわさんは　えきで　しんぶんを　かいます] 古川さんは駅で新間を買います。 4. Hanako is going to Kyoto (as well). [はなこさんは　きょうとに(も)　いきます] 花子さんは京都に(も)行きます。 5. Friends (will) watch a video at school. [ともだちは　がっこうで　ビデオを　みます] 友達は学校でビデオを見ます。 6. I (will also) write Kanji with a pencil. [わたしは　えんぴつで　かんじを　かきます] 私は鉛筆で漢字を書きます。 7. Friends of the teacher also came during the reading of this letter. [てがみを　よんでいるあいだ、せんせいの　ともだちも　きました] 手紙を読んでいる間、先生の友達も来ました。 8. I will continue learning Japanese tomorrow. [あした、わたしは　にほんごを　べんきょうし　つづけます] 明日、私は日本語を勉強し続けます。 === 良い一日をお過ごし下さい。

今回、私の答えを最も良い答えのように選んで下さって誠に有り難う御座いました。君に助けてあげて私の光栄です。良く勉強し続けてください。良い一日を。／Thank you very much for having chosen my answer as the best answer this time. It is my honor to help you. Please kindly keep up the good work. Good day.