忘れた (forgot) is the past form of 忘れる (forget). 忘れちゃった is the colloquial way of saying 忘れてしまった (have forgotten). No need to say, FORGET is one of the tricky English words, which doesn’t necessarily accord with the time tense of the sentences. Don’t let it confuse you.

No need to say, there is no adjective form of the word LIKE when to mean like-like. Also the word FAVORITE posses two usages; noun (this is my favorite), and adjective (this is my favorite music). 好きな is the adjective form of the verb 好き. If you say FAVORITE as a noun in Japanese, it should be 好きなもの. You know these right? The original message 好きな日本語は? seems to be too colloquial and missing the actual object of the sentence. Here 日本語 should’ve worked to modify the missing object 言葉. Your friend actually asked you what’s your favorite Japanese “word/phrase”; 好きな日本の言葉は何ですか. No wonder you got confused. This is the book I like. // これがぼくの好きな本。 This is my favorite book. // same as the above This is my favorite. // これがぼくの好きなもの。 The book I like is awesome. // ぼくが好きな(この)本は最高。 My favorite book is awesome. // same as the above

入学試験の申込締切を見過ごしてしまった

 

見過ごす means overlook or ignore, like to overlook one's injustice / 不正を見過ごす. You always have to notice it, and ignore it. 見落とす is a similar word, but you fail to pay attention and miss it. However even if you had applied 見落とす to this title, it may have sounded werid because you had just forgotten the deadline, right?

The differences of these are sort of philosophical, but here is a crux. 思う comes from passive attitude, and 考える from more active attitude. 寒(さむ)いと思う: perfect because you FEEL the cold. 寒いと考える: weird because you don't need to DEEM.

ともかく: When you do something less important before anything else. 仕事をしなくてはいけないが、今はともかく食べる。 Even though I have to work, I'm going to eat for now. とにかく: When you do something important before anything else. お腹は空いているが、今はとにかく仕事をする。 Even though I'm hungry, I have to work at any rate.

行かないといけない consists of 行かない + いけない, which is double-negative. If you've spent a lot of time with Japanese people, you've probably realized already that many of us use double-negative in English a lot because w-negative is pretty common in our language. As for 私は行け, we understand what you mean but unfortunately the saying is grammatically incorrect. It sounds like an order to yourself like Go Myself Now.

I ate serial today.

I enjoyed the breakfast.

I sometimes skip my breakfast.

 

きょう、私はシリアルを食べました。

私は朝食を楽しみました。

私は時々、朝食を抜きます。

 

穀物 means cerial grain as a whole. What you ate is シリアル we call. Also, when we skip a meal, we express it as 朝食/昼食/夕食 を抜く. If it's confusing, 時々、わたしは朝食を食べない is all fine.