Why in the second example we can't replace たら with ば but in the first one we use ば.
Aren't きてください and 連絡してください both requests or is there a difference?
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My impression is that ば presents a vague or hypothetical situation/state, and ら applies to actions and is similar to "when."
If tomorrow happens to be convenient, please come.
When you come to Tokyo, please contact me without fail.
With those implications, the opposite doesn't make much sense.
"When tomorrow is convenient, please call." The conditional is too strong and confident for the unknown situation.
"If perhaps you come to Tokyo, please contact me without fail." The conditional is too weak, and it is modifying an action.
I agree with Darcinon in general, but I'd add that the difference between ～ば and ～たら is not situation/state vs. actions, but rather hypothetical vs. probable. I will try to describe it in short.
X～たら、Y construct always implies, that X has occurred before Y, and
- either Y can/could occur only after X
- or Y is a result of x
It may help to know for better understanding, that ～たら form is derived form archaic form for complete action.
Examples of the case 1
先生に聞いたら、すぐ分かりました。 = If you listen to a teacher, you will understand this immediately.
花子さんは朝ご飯を終わったら、家を出てしまいました。 = When Hanako had finished her breakfast she left home.
Examples of the case ２
大学を出たら、小学校の先生になります。 = When I graduate, I will become an elementary school teacher.
危ないよ。 落ちたら、けがをするよ。 = Watch out! If you fall, you'll hurt yourself!
On the other hand, ～ば form just expresses a condition, that can be in this case hypotetical and even unreal
兄が行かなければ、私も行きませんよ。 = If my brother does not go, I will not go as well.
私が男であれば、サッカー選手になりたい。 = (a woman speaking) If I were a man, I'd want to be a football player.
So you can note, that ～たら and ～ば are not always interchangeable
金持ちになったら、親に家を買ってあげます。 = When I become a rich man I will buy my parents a house. (sounds like a real plan)
金持ちになれば、親に家を買ってあげます。 = If I became a rich man I would buy my parents a house. (sounds like a nice dream)
時間があれば、東京へ行きたい。 = If I have spare time I'd like to go to Tokyo. (OK)
時間があったら、東京へ行きたい。= ??? This sounds weird, something like "When I have spare time I will want to go to Tokyo (but without spare time I don't have such desire)".