Learn Hardcore Portuguese (Portugal): Ontem à noite vi relâmpagos sobre a colina durante a tempestade. - Last night I saw lightning over the hill during the storm.

The accent on à shows it’s a contraction of the preposition a + the feminine singular definite article a. In European Portuguese you always say à noite (“at night”), so a + a noite becomes à noite. Writing ontem a noite without the accent/article would be ungrammatical.

Can I say ontem de noite instead of ontem à noite?

In Portugal, the standard time-of-day expression is ontem à noite. Although you may see trabalho de noite (“I work at night”), when referring to “last night” you should use ontem à noite. Saying ontem de noite sounds awkward or non-standard.

Why is there no subject pronoun before vi?

Portuguese is a “pro-drop” language, meaning subject pronouns (like eu) are often omitted because the verb ending already indicates person and number. Vi is first-person singular of ver, so adding eu is optional and normally only used for emphasis: Eu vi.

What does relâmpagos mean, and why is it plural?

Relâmpagos are “lightning bolts” or “flashes of lightning.” In Portuguese you normally count each flash, so you use the plural if you saw more than one. If you refer to a single flash, you’d say vi um relâmpago. Note that “thunder” (the sound) is trovão, not relâmpago.

What does sobre mean in sobre a colina? Could I use em cima da colina or na colina instead?

Sobre generally means “over,” “above” or simply “on” something. Here relâmpagos sobre a colina implies the flashes appeared above or on the hill’s outline. You could say em cima da colina (“atop the hill”) or na colina (“on the hill”), but each has a nuance:

sobre a colina = over/above the hill
em cima da colina = right on top of it
na colina = somewhere on its surface

What does durante indicate in durante a tempestade? Could I use enquanto instead?

Durante is a preposition that means “during” plus a noun (tempestade). You cannot follow durante with a verb. Enquanto is a conjunction meaning “while,” and it must introduce a clause with a verb, e.g. enquanto chovia or enquanto a tempestade durava. So you choose based on whether you want noun or verb after.

Could I reorder the sentence elements, and would it change the meaning?

Yes. Portuguese word order is fairly flexible. For example:
• Vi relâmpagos ontem à noite sobre a colina durante a tempestade.
• Ontem à noite, durante a tempestade, vi relâmpagos sobre a colina.
All convey the same basic facts, but moving phrases can shift emphasis (time first, circumstances first, etc.). Just watch that modifiers stay next to what they modify to avoid ambiguity.