[Aeraaa]

There isn't any magical effect infantry has in the game (or IRL for that matter). Infantry's toughness comes from being hard to spot and forest enhances that trait. Simply put, it is quite often the case that an infantry squad ambushes a tank, fires off 2-3 AT munitions and is not even spotted. Bear in mind though, that in 2020, vehicles have better optics (many times even thermals) which mean they are better equipped to spot grunts.

Apart from that, most tanks in 2020 are practically immune from the front from any infantry carried AT weapon. That means you have to be sure you hit the flanks or even better, the rear of the vehicle. A good trick is to put at least two squads on opposite sides of a road (if the forest has a road running through it, which is where usually the tanks advance). When one tank passes, one squad hits it from the side. Tank will reply either by turning its turret or sometimes by turning the entire hull. Then you can use the other squad on the other side of the road to hit it from a more advantageous position (again, sides or rear, don't try the same thing to the front).

Most of time you need to have numbers (i.e. more rifle squads than enemy tanks) so that at least one squad will be in a position to fire a good shot. That is not a problem though, since tanks are much more expensive than infantry squads (by a factor of around 10-15 or so in 2020).