Try this...
Put the trans in neutral, engine idling. Engage the clutch - that means take your foot off the pedal.

Try to shift into each forward gear. Don't force it in. The synchro will prevent you from pushing the lever all the way into gear. If the synchro is weak you can push thru it & get it to grind. Don't try to put it in reverse, 'cause there's nothing to stop you & it'll just grind.

Now push (not too hard) against one of the synchros & then disengage the clutch. When the input shaft spins down to a stop, the synchro will let you into gear. That's how it's supposed to work... A weak synchro will let you push into gear before the input shaft is completely stopped.