If I have two Tideturners defending Forbidding Waste Battlefield and then the opponent attacks and uses Hextech Ray on one of them, will my surviving Tideturner have -2 might? I want to check if Forbidding Waste checks only at the start of combat or if it's always checking.

Yes, your surviving Tideturner will have the -2 Might penalty.

Forbidding Waste is a battlefield with a static ability that uses "while" language. According to the rules and established FAQs (such as FAQ #135), "while" abilities are static and continuously check their conditions. They do not check only at the start of combat; they are always active.

Sequence of Events

1. Initial State: You have two Tideturners defending at Forbidding Waste. Because there are two friendly units at the battlefield, the condition "defending alone" is not met. Neither Tideturner has the -2 Might penalty.
2. Opponent Action: The opponent plays Hextech Ray targeting one of your Tideturners.
3. Resolution: Hextech Ray deals 3 damage to the targeted Tideturner. Assuming it has 2 Might (its base value), it is killed and leaves the battlefield.
4. State Update: With one Tideturner gone, the remaining Tideturner is now the only friendly unit at Forbidding Waste.
5. Continuous Check: Because Forbidding Waste is always checking, it immediately detects that the remaining unit is now "defending alone."
6. Result: The static ability of Forbidding Waste immediately applies the -2 Might penalty to the surviving Tideturner.

Note: I could not fully verify this ruling against the retrieved rules as the card Forbidding Waste was not in the provided context.