Bidding
The auction decides who plays alone as declarer, and a bid is a promise: my hand is worth at least this much. Knowing when to pass, and when a marginal hand is still worth taking, is half the game before a single card falls.
Read the official bidding rules.
Ace and Ten of Diamonds: twenty-one Augen you'd rather not carry into a Ramsch round.
Knowing when to pass
When nothing in your hand comes close to covering even the lowest bid, pass immediately. It costs nothing; chasing a bid your hand can't make costs double if you lose it.
Too many Jacks for Ramsch
Everyone else has passed, and only you, as Forehand, can still take the game at 18 (Vorhandrecht) instead of letting the round fall to Ramsch. Holding three or more Jacks makes that a good trade even on a marginal hand: those same Jacks are exactly what makes a Ramsch round dangerous to sit through.
Too many points for Ramsch
The same Vorhandrecht call applies with 30 or more Augen sitting in Aces and Tens: that's a lot of value to end up holding if you're stuck taking tricks in a Ramsch round, where the biggest pile of Augen loses. Taking the game at 18 instead keeps that risk off the table.
Accepting a Null bid
A hand that's genuinely safe for Null doesn't need to hedge. Bid straight to what it's actually worth, no need to see the Skat first before committing.
A speculative Null
Sometimes a Null hand looks promising but isn't quite safe enough to commit to outright. Bidding low (18, well under the hand's real value) buys a look at the Skat first: if it helps, you're still playing Null; if it doesn't, you've risked less finding out.