We begin this sentence with the subject, remembering that Japanese does not have specific words for ‘mine/yours/etc.’ Since this is a simple ‘is/was’ sentence, we use the ‘XはYです’ construction, but in the past tense. We arrive at this sentence:
私の自転車は三万円でした。
わたしのじてんしゃは三まんえんでした。

We begin this sentence with the subject, remembering that Japanese does not have specific words for ‘mine/yours/etc.’ Since this is a simple ‘is/was’ sentence, we use the ‘XはYです’ construction, but in the past tense. We arrive at this sentence:
私の自転車は三万円でした。
わたしのじてんしゃは三まんえんでした。

‘きのう’ does have kanji associated with it, ‘昨日,’ but the hiragana form is used just as often.
Oftentimes we do not use ‘は’ with time words like ‘きのう,’ but we do here because we are talking about it specifically, not just about something that occurred at that time. Since this is a simple ‘is/was’ sentence, we use the ‘XはYです’ construction, but in the past tense. We arrive at this sentence:
昨日は日曜日でした。
きのうはにちようびでした。

‘きのう’ does have kanji associated with it, ‘昨日,’ but the hiragana form is used just as often.
Oftentimes we do not use ‘は’ with time words like ‘きのう,’ but we do here because we are talking about it specifically, not just about something that occurred at that time. Since this is a simple ‘is/was’ sentence, we use the ‘XはYです’ construction, but in the past tense. We arrive at this sentence:
昨日は日曜日でした。
きのうはにちようびでした。

We begin this sentence with the subject, just as we would in English. Since this is a simple ‘is/was’ sentence, we use the ‘XはYです’ construction, but in the past negative form. Remember that the past negative is slightly different in character than the present affirmative and negative. It is also different from the past affirmative. We arrive at this sentence:
山下先生は日本大学の学生じゃなかったです。
やましたせんせいはにほんだいがくのがくせいじゃなかったです。

We begin this sentence with the subject, just as we would in English. Since this is a simple ‘is/was’ sentence, we use the ‘XはYです’ construction, but in the past negative form. Remember that the past negative is slightly different in character than the present affirmative and negative. It is also different from the past affirmative. We arrive at this sentence:
山下先生は日本大学の学生じゃなかったです。
やましたせんせいはにほんだいがくのがくせいじゃなかったです。