Do monsters respawn in Half Life?

No; NPCs generally do not respawn in Half Life. I'm not aware of exceptions, so cleared areas should remain clear for your backtracking pleasure.

Monsters/Enemies respawn only at certain locations. ( Holds true for every difficulty level )

This is a list of locations sorted by game playtrough

• Office Complex (map c1a2) / Headcrabs

  The room with the flooded room and electricity. There is a ventilation shaft through you which have to process to advance, inside is a horizontal ventilator that is spinning ( it will kill you if you don't crouch). Above the ventilator is a hole with infinitely spawning headcrabs. They do always fall onto the ventilator and die.
  
  

• We've Got Hostiles ( map c1a3b ) / Marines

  The first time you get to the surface. The open area has marines, an V-22 Osprey and the area is constantly bombarded with artillery. Osprey is periodically dropping four marines behind electrified fence which you cannot get over. ( you can using grenade jump, but you are not supposed to ). As long as Osprey is operational and you keep killing marines, infinitely many will drop from it. You cannot collect infinite ammo as there is no effective way to kill the marines.
  
  

• We've Got Hostiles ( map c1a3c ) / Marines

  Just after the previous encounter. Open the shaft door, in the large vertical shaft with the large ventilator at the bottom. Two marines will constantly drop from the Osprey with the same rules as the marines on the surface before. This time you can collect the ammo, but it is very difficult to do it efficiently.
  
  

• Surface Tension ( map c2a5e ) / Marines / Infinite machine gun ammo

  At the end of this map inside a military base. Another Osprey is circling around a large asphalt area where are two Alien Grunts and is dropping infinitely many marines. This time you can get as much machine gun ammo as you wish. First kill the Alien Grunts, then go to the right side of the map. There is a large cannon. Aim it at the nearby wall, be careful not to aim to close as you will damage yourself, and weight down the fire button. Marines will gladly run into the explosions.
  Result after 10 minutes:

  

• Gonarch's Lair ( map c4a2a ) / Headcrabs

  The second stage of the fight with Gonarch ( headcrab queen ). In the middle is a chasm with three ledges and an crevice. Inside the crevice and in two of the ledges are infinitely spawning headcrabs. They spawn out of thin air if you are not very close ( stand on an ledge above the chasm ).

  

• Nihilanth ( map c4a3 ) / Alien Slave , Alien Controller

  The last boss fight in half life. Every couple of normal attacks, Nihilanth ( boss ) fires a homing green portal towards the player. Every time that portal is destroyed, either by hitting the player or hitting the wall, an alien slave ( vortigaunt ) or an alien controller is spawned. This will occur only if there are two or less monsters already present( not counting Nihilanth ).

  Trick: You can destroy the green homing portal using the Hornet Gun, to avoid being teleported away from the main fight.