Μόλις άνοιξα τον σύνδεσμο, το βίντεο έκανε λίγα δευτερόλεπτα να ανοίξει.

Here μόλις means as soon as.

So Μόλις άνοιξα τον σύνδεσμο... means As soon as I opened the link...

A useful thing to know is that μόλις can also mean just in other contexts, for example I just arrived. In this sentence, though, it introduces something that happened immediately before the next action.

Άνοιξα is the aorist form of ανοίγω, meaning I opened.

Greek uses the aorist here because the speaker is talking about a single completed action in the past: opening the link. It is not describing an ongoing action like I was opening; it is a completed event.

Because τον σύνδεσμο is the direct object of the verb άνοιξα.

• ο σύνδεσμος = the link as the subject
• τον σύνδεσμο = the link as the object

Since the speaker opened the link, the noun has to be in the accusative case, so ο σύνδεσμος becomes τον σύνδεσμο.