hello,
how my I make a video fadein transition blocking it on a 50% of transparency?
thanks, bye.

Not exactly sure what you want to achieve, but what about using either an Affine or Composite transition on a clip and setting its Opacity to 50%? Make sure that you have either another video clip or a black clip on the track below. Otherwise Opacity doesn't work for transitions.

on a background with the sea and the sky I want to appear gradually the face of a woman but that remains very hazy, just see. Then after three seconds must disappear again.
I hope to be clear, I'm italian and my english is'nt very well. sorry.

That's now clear, no need to excuse.

Place the background clip on, say, track 2. Place the woman's face on the track immediately above, track 1. Add a transition of type Affine or Composite to the top clip on track 1. Resize clip and transition to proper length as you need.

Make sure that the face was shot against a black background. Otherwise you need to add more tricks I can't help you with without further information.

Select the transition and place the playhead position to the beginning of the transition. There's kind of a time line in the transition dialog, and you'll see a few buttons immediately below it. Press the << button, so the playhead goes to the beginning of the transition. Now set Opacity to 0. The womens face disappears and the background becomes visible.

Say you want to fade in to 50% over 5 seconds. So position the playhead at 0:00:05:00. Then press the + button below the time line in the transition dialog. This creates a new keyframe. Set Opacity to 50%, that is, 50 if my memory serves me right.

Then set another keyframe where you want to start fading out. This will automatically take over 50%.

Use the >> button in the transition dialog to move to the end of the transition. Press + again, then set Opacity to 0 for this final keyframe.

Done. Play to check.

This may sound much more difficult thn it actually is. You will quickly get accustomed to working with key frames. And transitions.