If a creature has been slowed either from the slow spell or a stone golem's slow ability, it can't make more than one attack per turn. If a character can make multiple attacks per turn however (eg. via extra attack) they can normally substitute a shove or a grapple in place of one of those attacks. Can a character capable of multiple attacks per turn, who is currently slowed, make a single weapon attack and then use their remaining attacks to shove/grapple?
Grapples and shoves are attacks; you only get one attack while affected by slow
The slow spell states:
[...] Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn [...]
When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you're able to make multiple attacks with the Attack action, this attack replaces one of them [...]
Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you. If you're able to make multiple attacks with the Attack action, this attack replaces one of them [...]
This shows us that both grappling and shoving are considered attacks, and thus the slow spell also prevents you from making multiple grapples or shoves, or even a regular attack and a grapple/shove. You only get one attack, total, for your turn, this includes regular attacks, grapples, and shoves.
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