Choosing たら, と, なら, or ば
Japanese conditionals do not all express the same kind of 'if.' Choose according to the expected result, the sequence of actions, and any situation introduced by the other person.
たら: once a condition is met
雨が降ったら、出かけません。 means 'If it rains, I will not go out.' Add ら to the ta-form. This can suppose a future situation; it does not assert that rain has already fallen.
家に着いたら、連絡してください。 asks someone to get in touch after arriving home. A request or intention can follow たら. Notice the order between meeting the condition and performing the later action.
と: a regular or expected consequence
このボタンを押すと、画面が変わります。 explains that pressing this button changes the screen. と commonly connects a condition with a regular consequence, such as a machine's response or a recurring result.
In a basic future conditional, do not simply put a request such as 'contact me when you arrive' after と. However, と also has uses for discoveries and sequences of past events, so regular consequences are not its only function.
The focus of なら and ば
京都に行くなら、この本を持って行ってください。 means 'If you are going to Kyoto, take this book.' なら can take up someone's plan as the basis for advice. Taking the book can happen before reaching Kyoto.
時間があれば、手伝います。 means 'If I have time, I will help.' Forms include ある → あれば and 安い → 安ければ. Whether intentions or requests fit after ば depends on the first clause and the participants; avoid mechanical substitution.
Check: For 'Please contact me when you get home,' is 着くと or 着いたら more natural?
Show the answer and why
家に着いたら、連絡してください。 is natural. It requests an action after arrival, so do not replace it with a basic と conditional.