הניסיון הראשון היה קשה, אבל מהניסיון השני למדנו הרבה.

It can mean either, depending on context. In this sentence, because it says הראשון and השני (the first, the second), it most naturally means attempt, try, or trial.

So here הניסיון הראשון is best understood as the first attempt, and הניסיון השני as the second attempt.

Because the phrase is definite: the first attempt, not just a first attempt.

In Hebrew, if the noun phrase is definite, the noun takes ה־:

• ניסיון ראשון = a first attempt / first attempt
• הניסיון הראשון = the first attempt

So the ה־ shows that we are talking about a specific attempt.

Because in Hebrew, adjectives usually come after the noun they describe.

That includes ordinal numbers like ראשון (first) and שני (second):

• הניסיון הראשון = literally the attempt the-first
• הניסיון השני = literally the attempt the-second

This is normal Hebrew word order.

In Hebrew, when a noun is definite and it has an adjective, the adjective is usually definite too.

So:

• הילד הטוב = the good boy
• הניסיון הראשון = the first attempt
• הניסיון השני = the second attempt

This is different from English, where only the appears once. In Hebrew, definiteness is often shown on both the noun and the adjective.

היה means was, and קשה means difficult / hard.

The subject is הניסיון הראשון, and ניסיון is a masculine singular noun, so the past form of to be is masculine singular: היה.

So:

• הניסיון הראשון היה קשה = The first attempt was difficult

Also, in Hebrew, adjective predicates like קשה usually do not take ה־ here. It is simply describing the subject: was difficult.

Because Hebrew verbs often already include the subject.

למדנו means we learned:

• למד = learned
• ־נו = we

So למדנו already contains we. You could add אנחנו for emphasis, but it is not necessary:

• למדנו הרבה = we learned a lot
• אנחנו למדנו הרבה = we learned a lot (with extra emphasis on we)

Because the preposition מ־ (from) is commonly attached directly to the following word in Hebrew.

So:

• מ־
  • הניסיון → מהניסיון

This is very common with short prepositions in Hebrew, such as:

• בבית = in a house / in the house depending on context
• לילד = to a boy / to the boy
• מהניסיון = from the experience / from the attempt

Both are related. The full preposition is מן (from), but in normal Hebrew it is very often shortened to מ־.

So:

• מן הניסיון = a fuller form
• מהניסיון = the usual contracted form

With the definite article ה־, the combination becomes a fused form. In pointed Hebrew you may see something like מֵהַניסיון, but in everyday unpointed writing you usually just see מהניסיון.

Here הרבה means a lot.

In למדנו הרבה, it works like an adverb:

• למדנו הרבה = we learned a lot

It does not mean many in the sense of directly describing a plural noun here. If you wanted to say we learned many things, you could say something like:

• למדנו הרבה דברים

But by itself, למדנו הרבה is a very natural way to say we learned a lot.

Hebrew word order is flexible, and moving a phrase to the front often gives it emphasis.

So:

• מהניסיון השני למדנו הרבה puts emphasis on from the second attempt
• למדנו הרבה מהניסיון השני is also correct, and may sound a little more neutral in some contexts

In this sentence, putting מהניסיון השני first helps create a contrast with the first half:

• The first attempt was difficult, but from the second attempt we learned a lot

Yes. That is also correct and natural.

The difference is mainly emphasis:

• אבל מהניסיון השני למדנו הרבה = stronger focus on from the second attempt
• אבל למדנו הרבה מהניסיון השני = a more neutral order

Both mean essentially the same thing. Hebrew often allows this kind of variation more freely than English does.