Στο γιαούρτι βάζω λίγο φιστίκι και λίγο μέλι.

Στο is the contracted form of σε + το.

So:

• σε = a very common preposition meaning in, on, at, to
• το = the for a neuter singular noun

Together, στο γιαούρτι means in the yogurt here.

Greek σε is broader than any one English preposition, so depending on context it can be translated as in, into, on, at, or to.

Because in Modern Greek, σε often combines with the definite article:

• σε + το = στο
• σε + τη(ν) = στη(ν)
• σε + τα = στα

So στο is not a separate dictionary word with a totally different meaning; it is just the normal contracted form.

After σε, Modern Greek normally uses the accusative.

So in στο γιαούρτι, γιαούρτι is in the accusative.
But γιαούρτι is a neuter noun, and for most neuter nouns, the nominative and accusative singular look exactly the same.

So:

• dictionary form: το γιαούρτι
• after σε: στο γιαούρτι

The noun itself does not visibly change.

Βάζω literally means I put. In food contexts, it often means I add.

So here it can be understood as:

• I put
• I add

It is first person singular, present tense.

Depending on context, the Greek present can mean either:

• I add / I usually add
  or
• I’m adding

Λίγο means a little or some here.

It is the neuter singular form of λίγος and matches the nouns it goes with:

• φιστίκι = neuter singular
• μέλι = neuter singular

With food substances or ingredients, Greek often uses this form the way English uses a little or some.

So:

• λίγο μέλι = a little honey / some honey
• λίγο φιστίκι = a little pistachio / some pistachio

Greek often repeats a quantity word when it applies separately to two different things.

So:

• λίγο φιστίκι και λίγο μέλι

sounds clear and natural: each ingredient has its own amount.

If you removed the second λίγο, the sentence might still be understood, but repeating it is more explicit and very normal Greek.

Because here it is being treated more like an ingredient or substance, not as individually counted nuts.

So:

• λίγο φιστίκι = some pistachio / a little pistachio, in a general ingredient sense
• λίγα φιστίκια = a few pistachios, focusing on separate countable nuts

English does something similar sometimes:

• some almond
• some peanut
• some chocolate

when talking about ingredients or flavoring, not individual items.

Because Greek often leaves out the article with indefinite amounts of food or substances.

So:

• λίγο μέλι = some honey
• λίγο φιστίκι = some pistachio

If you used the article, it would sound more specific:

• το μέλι = the honey
• το φιστίκι = the pistachio / the pistachio stuff

In this sentence, the speaker is just naming ingredients, so no article is needed.

Yes. Greek word order is fairly flexible.

For example, you could also say:

Βάζω λίγο φιστίκι και λίγο μέλι στο γιαούρτι.

That is also perfectly natural.

Starting with Στο γιαούρτι puts the focus first on the yogurt, almost like:

• As for the yogurt, I add...
• In the yogurt, I put...

So the original sentence gives a bit of emphasis to where the ingredients go.

Greek σε / στο is broad enough to cover several English prepositions.

With βάζω, στο γιαούρτι could be understood as:

• in the yogurt
• into the yogurt
• sometimes even what English might naturally describe as on the yogurt, depending on the situation

Greek often uses σε / στο where English makes a finer distinction.

A rough pronunciation is:

Sto ya-OOR-ti VA-zo LEE-gho fee-STEE-kee ke LEE-gho ME-lee.

A slightly more exact version in IPA is:

[sto ʝaˈurti ˈvazo ˈliɣo fiˈstici ce ˈliɣo ˈmeli]

A few helpful notes:

• γιαούρτι sounds roughly like ya-OOR-ti
• βάζω = VA-zo
• λίγο = LEE-gho
• και is often pronounced like ke in normal speech
• μέλι = ME-lee