Transmission keeps popping out first gear

Hello OCTAVIAN.

As a first step check the selector lever if its working and engaging all the gears properly with engine off. if it is okay then adjustment and the selector lever links are good. problem is probably from the gearbox inside that has small metal ball with spring that slides into a groove and locks the fork link in position, when excessive wear caused between the ball and the groove or weak spring the gear will jump out from its position.
This is our guide how to remove and reinstall gearbox:
https://www.2carpros.com/articles/transmission-replacement-removal-manual

Hello, it is a basic manual transmission I don't think it needs special tool. I found exploded view of front wheel drive gearbox you can have an idea what inside it. Check fig 1, first after you drain gearbox oil remove the rear cove and transmission axle cover fig 2. You will see the gear assemblies input and out shaft fig 3 and fig 4, on the input shaft biggest gear must be the first gear, fig 5 you can see the selector shift fork assembly and the steel ball with its spring that seems your problem is there. Gear with the small point which is the metal ball that when the gearbox is disassembled clearly defect must be appeared at the selector link for the first gear.

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