Learn Hardcore Japanese: tenki ga warui toki, watasi ha zitensya de ha naku tikatetu ni norimasu. - When the weather is bad, I take the subway instead of a bicycle.

In subordinate clauses (like time clauses with とき), が typically marks the neutral subject of a descriptive phrase. Using は here would add contrast or emphasis on “the weather,” which isn’t intended. So 天気が悪いとき simply means “when the weather is bad” without extra nuance.

How does とき work in 天気が悪いとき? Why is it attached directly to 悪い?

とき means “time” or “when.” When you attach it to verbs or adjectives, you use their plain (dictionary) form directly before とき. Thus 天気が悪い (weather is bad) + とき = “when the weather is bad.”

What is the function of ではなく in this sentence? Could I also say ではなくて or じゃなくて?

ではなく is the contrastive negative form of である, meaning “not A but B.” Here it contrasts 自転車 (bicycle) with 地下鉄 (subway): “not by bicycle but by subway.” For a smoother link to the next phrase, you can use ではなくて. In casual speech, speakers often say じゃなくて or even just じゃなく.

Why is で used after 自転車? What does it indicate?

The particle で marks the means or method. 自転車で literally means “by bicycle” or “using a bicycle.”

Why is に used after 地下鉄 instead of を in 地下鉄に乗ります?

Japanese uses に乗る for boarding or riding vehicles. So you always say 電車に乗る, バスに乗る, 地下鉄に乗る, not ～を乗る.

Why is the verb 乗ります in the present tense? Does it refer to the future or something else?

Japanese non-past (present) tense covers both habitual actions and general truths. Here it expresses a habitual action: “whenever the weather is bad, I ride the subway.”

Can I omit 私は in this sentence?

Yes. Japanese often drops the subject when it’s understood. You could simply say 天気が悪いとき、自転車ではなく地下鉄に乗ります。 Including 私は adds clarity or mild emphasis on the speaker.

How would I say this more casually?

A casual, plain-speech version could be:
天気が悪いとき、自転車じゃなくて地下鉄に乗る。
Here じゃなくて replaces ではなく, and 乗る is the plain verb form instead of polite 乗ります.