Sve što trebamo za svibanj jest dobar plan i malo sunčanog vremena.

Sve što means everything that or all that.

So:

• sve = everything / all
• što = a relative word meaning that / what in this kind of structure

In the sentence, Sve što trebamo za svibanj means Everything that we need for May.

This is a very common Croatian pattern:

• Sve što znam = Everything that I know
• Sve što želiš = Everything that you want

Trebamo is the 1st person plural present tense of trebati.

The present-tense forms are:

• trebam = I need
• trebaš = you need
• treba = he/she/it needs
• trebamo = we need
• trebate = you (plural) need
• trebaju = they need

So što trebamo means that we need.

Because Croatian often drops subject pronouns when they are already clear from the verb ending.

The ending -mo in trebamo already tells you the subject is we.

So Croatian usually says:

• trebamo rather than mi trebamo

You can add mi if you want emphasis, but it is not necessary.

Za svibanj means for May.

• za = for
• svibanj = May

Here it means something like for the month of May or for May’s needs/plans.

A useful contrast:

• za svibanj = for May
• u svibnju = in May

So this sentence is not mainly saying during May, but rather what we need for May.

Because za here takes the accusative case, and for this noun the accusative singular has the same form as the nominative singular.

So:

• nominative: svibanj
• accusative: svibanj

That is normal for many masculine inanimate nouns in Croatian.

Jest is the full form of is, while je is the short clitic form.

So both are forms of the verb biti (to be):

• je = short, very common
• jest = full, more formal, more emphatic, sometimes stylistically clearer

In this sentence, jest sounds a bit more formal or polished:

• Sve što trebamo za svibanj jest dobar plan...

You may also hear:

• Sve što trebamo za svibanj je dobar plan...

That is also natural in everyday Croatian.

Because it is the predicate complement after the verb jest.

The phrase is in the nominative:

• dobar = masculine singular nominative
• plan = masculine singular nominative

So dobar plan means a good plan.

After biti (to be), Croatian very often uses the nominative for the thing identified as the subject’s complement.

Because malo meaning a little / a small amount of normally requires the genitive.

So:

• malo vremena = a little time / a little weather
• not malo vrijeme

Since vrijeme goes into the genitive singular, its adjective must match it:

• nominative: sunčano vrijeme
• genitive: sunčanog vremena

That is why the sentence has:

• malo sunčanog vremena = a little sunny weather

This is a very important Croatian pattern:

• puno vode = a lot of water
• malo novca = a little money
• dosta vremena = enough / a lot of time

Because vrijeme is in the genitive singular after malo.

Its forms are:

• nominative singular: vrijeme
• genitive singular: vremena

So:

• vrijeme = weather / time
• malo vremena = a little weather / a little time

In this sentence, because of sunčanog, it clearly means weather, not time.

Here it means weather.

Croatian vrijeme can mean both:

• time
• weather

Context tells you which meaning is intended. In this sentence, sunčanog vremena can only really mean sunny weather, so the meaning is clear.

Because the grammatical subject is not that final list. The subject is the whole idea:

• Sve što trebamo za svibanj = Everything that we need for May

That whole subject is treated as a single thing/idea, so the verb is singular:

• jest

Then the sentence tells you what that everything consists of:

• dobar plan i malo sunčanog vremena

This works much like English:

• All we need is a good plan and some sunny weather.

Even though two items follow, all we need is still grammatically singular.

Yes, and that is a very useful thing to notice.

Croatian often expresses need in two common ways:

1. trebati with a normal subject:

   • Trebamo plan. = We need a plan.

2. trebati in an impersonal-style pattern with an indirect object:

   • Treba nam plan. = literally A plan is needed to us, meaning We need a plan.

So these are both natural:

• Sve što trebamo za svibanj...
• Sve što nam treba za svibanj...

The original sentence uses the first pattern, but learners should recognize both.

Yes, Croatian word order is fairly flexible, though not completely free.

The given version:

• Sve što trebamo za svibanj jest dobar plan i malo sunčanog vremena.

puts strong focus on everything that we need.

Other possible versions might be:

• Za svibanj trebamo samo dobar plan i malo sunčanog vremena.
• Sve što trebamo za svibanj je dobar plan i malo sunčanog vremena.

The exact emphasis changes a bit, but the core meaning stays the same.

Yes: in Croatian, names of months are normally written with a lowercase letter, unlike in English.

So Croatian writes:

• svibanj
• lipanj
• rujan

not Svibanj unless it begins a sentence.

That often surprises English speakers, because English capitalizes month names.