Learn Hardcore Chinese: jīntiān wǒ bù kāi chē, wǒ zuò dìtiě. - I'm not driving today; I'm taking the subway.

不 negates habits, general truths, or planned/arranged actions: 今天我不开车 = I’m not going to drive today / I don’t drive today (as a plan).
没 negates completed events in the past: 今天我没开车 = I didn’t drive today (after the fact).
You generally don’t combine 不 directly with a verb-perfect 了.

Is there any tone sandhi I should watch out for here?

不 (bù) changes to bú before a fourth tone. Here it’s before 开 (kāi, 1st), so it stays bù: bù kāi.
If you said 我不坐地铁, that would be wǒ bú zuò dìtiě because 坐 (zuò) is fourth tone.
Third-tone sandhi applies only before another third tone, which doesn’t occur in this sentence.

Does 开车 literally mean “open the car”?

No. 开车 (kāi chē) is a verb–object phrase meaning “to drive (a car).” 开 often means “operate/start” in such phrases:

开门 open the door
开电视 turn on the TV
开会 hold a meeting

Why is 坐 used with 地铁? Doesn’t 坐 mean “to sit”?

坐 is the default verb for traveling by public transport: 坐地铁, 坐公交, 坐火车. Alternatives:

Formal: 乘 / 乘坐地铁
Taiwan usage: 搭地铁 / 搭捷运

Do I need to repeat 我 in the second clause?

Not required if the subject is the same. All of these are fine:

今天我不开车，坐地铁。
今天我不开车，我坐地铁。 (more explicit/balanced)
To emphasize “instead,” use 而是: 今天我不开车，而是坐地铁。

Should there be a conjunction like “instead,” “so,” or “but”?

Chinese often just puts clauses side by side with a comma. Add a conjunction if you want a specific relation:

Contrast: 而是
Cause: 因为…所以…
Contrast/turn: 但是/不过

Does 今天 apply to both clauses or only the first?

It applies to the whole sentence here, i.e., both 不开车 and 坐地铁 happen today. Topic-first placement lets it scope over what follows.

Can I say 不会开车 to mean “won’t drive today”?

No. 不会开车 usually means “cannot/doesn’t know how to drive.” To say “won’t (don’t plan to) drive today,” use 今天我不开车. If you must express inability today (not skill), use 今天我不能开车.

Do I need something like 会 or 要 to show the future?

Not necessary—time words like 今天 already set the timeframe.

Intention/plan: 今天我要坐地铁 or 我打算今天坐地铁.
Prediction/probability: 今天我会坐地铁 can work, but note that 不会开车 is ambiguous (usually “can’t drive”), so for negation prefer 今天我不开车.

Can I add 了? What changes?

Sentence-final 了 marks a change of state or a new situation: 今天我不开车了 = I’m not driving today anymore / I’ve decided not to drive.
Verb-suffix 了 marks completion: 我开车了 = I’ve started driving / I drove (already). You don’t say 不…了 with this 了; use 没 for past negation: 我今天没开车.

Why is there a comma between the two parts?

Chinese commonly uses ， to join two closely related clauses. You could also write two sentences:

今天我不开车。我坐地铁。 Using ， keeps the contrast tight and natural.

Do I need measure words here?

Not in 开车 or 坐地铁. But when counting:

Car: 一辆车
One subway trip: 一趟地铁
Number of stops: 坐三站地铁

How do I say “take the subway to X” or add a destination?

我坐地铁去公司。
我坐地铁上班。
Place the destination after 去/到: 我坐地铁到市中心。

Is 和 used to connect these two clauses?

No. 和 links nouns/NPs, not full clauses in this way. For clauses, either use a comma or a proper conjunction like 而是, 但是, 所以, depending on meaning.

Could I just say 开 without 车?

Sometimes context allows it, e.g., 我今天不开了 (if everyone knows you mean driving). In general, 开车 is clearer; 开 alone is versatile and can mean many things (open, start, operate), so include 车 unless context is crystal clear.

What are regional or register alternatives for 地铁?

Mainland: 地铁
Taiwan: 捷运 (jiéyùn); you’ll hear 搭捷运
Formal/written: 乘地铁 / 乘坐地铁

What verbs go with other transport modes?

开车 drive a car
坐地铁/坐公交/坐火车 take the subway/bus/train
打车 / 打的 take a taxi (hail a cab)
骑自行车/骑电动车 ride a bicycle/e-bike
坐飞机 take a plane