In blackjack, you should hit (ask for another card) primarily in these situations:

• When your hand total is 11 or less, since you cannot bust by taking a hit.
• When your hand total is between 12 and 16 and the dealer's upcard is 7 or higher (7, 8, 9, 10, or Ace), as the dealer likely has a strong hand.
• When you have a soft hand (a hand with an Ace counted as 11) such as soft 17 or soft 18 and the dealer has a strong upcard, because hitting won't bust you and it could improve your hand.
• When you have a 12 or 13 against a dealer's 2, because the dealer might improve to a strong hand.

You should avoid hitting when:

• Your total is 17 or more (hard 17 or above).
• The dealer has a weak upcard (2 through 6), as the dealer is likely to bust and you should stand to avoid busting yourself.
• When strategy advises you to surrender, double down, or split instead.

In general, hitting is a strategy to improve a low or risky hand especially when the dealer is showing a strong card. Standing is safer when you already have a reasonably strong total or when the dealer is likely to bust with a weak upcard.