Learn English – begin a sentence with, “Unless, that is ….”

1. It's perfectly OK to begin a sentence with a conjunction. Just don't do it over. And over. And over. (Except for rhetorical or narrative effect. Or in translating the Hebrew Bible.)
2. Never put a comma after a conjunction: a comma is a “disjunction”, and defeats the purpose of the conjunction. I grant that you will find some authorities conceding that the first of a pair of commas enclosing a parenthetical phrase may follow a conjuction immediately. Formally, they're correct; but if the phrase is brief the comma-pair is unnecessary, and, because if the phrase is long it tends to blur the reader's sense of your syntax, it's bad practice to put it immediately after the conjunction.

I have seen a general increase in the frequency of this CONJ+comma construction over the last ten years, largely in business writing. I suspect there are two causes:

1. Residual discomfort with violating the “rule” against starting a sentence with a conjunction — people who are insecure are more likely to grasp at a misuse as a solution. It's a sort of hypercorrection.

2. An effort to achieve a colloquial emphasis on the conjunction — the comma isolates the conjunction and reproduces the pause in speech. In cases where this is aimed at, I suggest using a colon instead of a comma; the colon is fundamentally conjunctive. And the rarity of the device serves to isolate and emphasize the conjunction.

   I honor these users' attempts to write more colloquially. But: I deprecate the awkward means they employ.

Because if it rains tomorrow, I will get wet, I hoped for a sunny day.

Okay. It's fine-ish.

The tense seems a bit odd. You are talking about a possibility for the future (relative to now?) and of a hope in the past.

That's not wrong. It would make sense for example if someone had just told you (today) that there was a forecast of a strong chance of rain. The rain remains just a future possibility, but the hoping was done in the past, so all the tenses work logically.

It's just not a very likely combination of tenses, so standing on its own, without the context of such a forecast, it strikes me as odd, where it wouldn't just after such a forecast or some other reason for the unusual tense combination.

So let's change the tenses just so I'm no longer distracted by that!

Because if it rains tomorrow, I will get wet, I hope for a sunny day.

Can I follow that? Yes.

Is my understanding of if what you wanted it to be? I'm pretty confident it is.

Do I at any point get tripped up, and have to mentally backtrack to understand you? No.

Can I see any grammatical problems? The first comma isn't wrong, but isn't necessary either, and I think you'd be better of without it, but I don't see any problems.

Can you start a sentence with two subordinating conjunctions? I'd say "yes" as a general point of theory, and I'd accept the above as evidence to back it up.

Is it a good sentence? Ah…

It takes me a while to get to your point.

Now, that's not in a terrible way. I don't get half-way through your sentence, realise I misinterpreted a clause and have to go back and start again.

But it's generally not a good thing either. There's a lot of build-up to just the hope of a sunny day.

But it's not always a bad thing.

It happens with any Because sentence, and the if adds to the effect. Consider:

Because the Origami didn’t work

We gave you dancing lessons

Because the dancing lessons made you bored

We bought you a guitar

Because you said

It made your fingers sore

We found you a pony

Because the pony broke your leg and ran away

We got you drums

Here the fact that putting the subordinate clause first leads us along for a bit before we get to the main clause is the whole reason this song works.

Your "Because if" structure increases the "leading along" quality, but the pay-off doesn't seem to be worth it.

And of course example sentences are rarely those with the best punch. (It's hard enough to think of a good sentence when writing, or a valid example when examining grammar, so doing both together is really tough). The actual form of "Because if…" I'd say is fine, I'd just better get something after it's done leading me through the subordinate clause.