Learn Swedish: Vakten öppnade grinden klockan sju. - The guard opened the gate at seven o'clock.

In Swedish, you do not need a preposition for exact clock times. While English uses 'at', Swedish just says klockan sju. If you were to add vid (like vid klockan sju), it would mean 'around seven o'clock' rather than exactly at seven.

Why use grinden here instead of porten for 'the gate'?

They refer to different types of gates. En grind is a lighter outdoor gate, like one in a fence, garden, or driveway. En port is a heavier entrance gate or door attached to a physical building, such as an apartment block entrance or a castle gate.

Will native speakers actually pronounce the whole word öppnade?

In careful speech, yes, but in everyday spoken Swedish, the final -de on Group 1 past tense verbs is very frequently dropped. The word will often sound exactly like the infinitive öppna. You have to rely on the context of the conversation to know it happened in the past.