wǒmen shì zuò dìtiě qù yīyuàn de, piào yě bú guì.

It highlights or “focuses” a detail about a specific event, usually in the past. Here it emphasizes the manner of going: 坐地铁 (by subway). The focused detail sits between 是 and 的:

• Pattern: (主语) 是 + [time/place/manner/companion] + 动作 + 的
• Example: 我们是坐地铁去医院的。 (focusing on how we went)

That final 的 closes the 是…的 focus construction. It’s not the possessive 的. If you remove 是, you should also remove that 的. For example:

• With focus: 我们是坐地铁去医院的。
• Neutral statement: 我们坐地铁去医院。

贵 is an adjective (a stative verb), so it’s typically negated with 不: 不贵. Use 没(有) to negate actions, existence, or past experience (or in the set pattern 没那么 + adj). Both are possible in different patterns:

• 票也不贵。 (not expensive)
• 票也没那么贵。 (not that expensive)

也 means “also/too,” adding another related point. It comes after the subject/topic and before the predicate:

• 票也不贵。 (As for the tickets, they weren’t expensive either.) You cannot put 也 at the end or before the subject.

When speaking generally or collectively (“the tickets” as a category for that trip), Chinese can omit a measure word. If you want to count or be specific, add one:

• 两张票不贵。 (Two tickets weren’t expensive.)
• 地铁票也不贵。 (Subway tickets/fare weren’t expensive.)

坐 is the most common, neutral verb for “take (a vehicle)” in daily speech: 坐地铁/坐公交/坐火车. Alternatives:

• 乘/乘坐 (chéng/chéngzuò): more formal.
• 搭 (dā): common in Taiwan and some southern regions.
• Taxis: 打车/坐出租车.

Both can work:

• 去医院 focuses on the action “go (to).”
• 到医院 highlights the arrival at the destination. With 是…的, both are acceptable: 我们是坐地铁去/到医院的. The nuance difference is minor here.

Yes; that’s a core use. Examples:

• 我们是昨天坐地铁去医院的。 (time)
• 我们是从市中心坐地铁去医院的。 (starting place)
• 我们是和朋友一起坐地铁去医院的。 (companion)

Yes. 不是…的 denies a specific assumption:

• 我们不是坐地铁去医院的。 (We didn’t go by subway.) You can then add the correct detail: 我们是打车去医院的。 (We went by taxi.)

Yes, to make the addition explicit:

• 我们是坐地铁去医院的，而且票也不贵。 It’s also fine as two clauses separated by a comma, as in the original.