～たら indicates a condition under which a future event or state will happen or come true. The word もし(if) at the beginning of the sentence emphasizes that it is hypothetical(i.e., a particular thing will happen only if something else were to happen first).
もし、あした 雨が ふったら、ハイキングに 行きません。
If it rains tomorrow, we are not going hiking.
(もし、)あした 天気が 良かったら、買物に 行って ください。
If the weather is fine tomorrow, please go shopping.
(もし、)その もんだいが ふくざつだったら、こたえられません。
If the question is complicated, I cannot answer it.
(もし、)金持ちだったら、高い 物が 買えます。
If I were rich, I would be able to buy expensive things.
(もし、)わたしの 家に 車が なかったら、ふべんでしょう。
If my family didn't have a car, it would probably be inconvenient.
(もし、)むずかしい もんだいで なかったら、こたえられます。
If it isn't a difficult question, I can answer it.
