Hétfő óta az ültetőládában van a petrezselyem.

Óta means since in time expressions.

So:

• hétfő óta = since Monday

It is used after a point in time and shows that something started then and has continued up to now.

Examples:

• reggel óta = since morning
• tavaly óta = since last year
• kettő óta = since two o’clock

In this sentence, it tells us that the parsley has been in the planter box continuously since Monday.

Because the meanings are different:

• hétfőn = on Monday
• hétfő óta = since Monday

So:

• Hétfőn az ültetőládában van a petrezselyem.
  = On Monday the parsley is in the planter box.

• Hétfő óta az ültetőládában van a petrezselyem.
  = The parsley has been in the planter box since Monday.

The sentence you have is about duration from Monday until now, not just one day.

Ültetőládában means in the planter box.

It can be broken down like this:

• ültetőláda = planter box / planting box
• -ban / -ben = in

So:

• ültetőládában = in a planter box / in the planter box

Hungarian often packs the meaning of an English preposition into a noun ending instead of using a separate word like in.

This is due to vowel harmony, a very important Hungarian pattern.

The suffix meaning in has two forms:

• -ban
• -ben

Words with mostly back vowels usually take -ban.
Words with mostly front vowels usually take -ben.

Láda contains back vowels, so:

• ládában

Since ültetőláda ends with láda, the whole word takes:

• ültetőládában

Hungarian has two forms of the definite article:

• a
• az

Use:

• a before a consonant sound
• az before a vowel sound

Because ültetőládában begins with the vowel ü, Hungarian uses:

• az ültetőládában

Compare:

• a kertben = in the garden
• az ablakban = in the window

That is a very common and very good question.

Hungarian usually omits van in simple present-tense sentences when one noun or adjective describes another:

• A ház nagy. = The house is big.
• Ő orvos. = He/She is a doctor.

But van is normally used when talking about location or existence:

• A könyv az asztalon van. = The book is on the table.
• Péter otthon van. = Péter is at home.

Since this sentence says where the parsley is, van is needed:

• az ültetőládában van

Hungarian word order is flexible and often reflects topic and focus, not just fixed grammatical position.

This sentence has a natural, neutral structure where:

• Hétfő óta sets the time frame
• az ültetőládában gives the location
• van a petrezselyem presents the situation

A learner coming from English may expect a fixed order like The parsley has been in the planter box since Monday, but Hungarian often puts the time expression first very naturally.

Other orders are possible, but they can shift emphasis. For example:

• A petrezselyem hétfő óta az ültetőládában van.
  This is possible, but sounds more marked or contrastive.

So the given order is quite natural.

A petrezselyem means the parsley.

Hungarian often uses the definite article when talking about a specific, identifiable thing in context. In this sentence, we are presumably talking about a particular parsley plant or parsley that both speaker and listener can identify.

So:

• a petrezselyem = the parsley

If you said just petrezselyem, that would sound more like a bare noun, and it would not fit this sentence as naturally.

It only looks that way. Petrezselyem is simply the normal dictionary word for parsley.

So here:

• petrezselyem = parsley

It is not functioning as a possessive form in this sentence.

This is one of those words where the full noun just happens to end in something that resembles another ending. Learners often notice this because Hungarian really does use endings like -m / -em / -om for possession elsewhere, but in this case the whole word is just lexical vocabulary.

Because it is the subject of the sentence.

In Hungarian, the basic subject form is the nominative, and that usually has no ending.

Here:

• a petrezselyem = the parsley → subject
• az ültetőládában = in the planter box → location

So the location noun gets the -ban ending, but the subject noun stays in its basic form.

Yes — in English, that is often the most natural translation.

Literally, Hungarian has present tense:

• van = is

But because of óta (since), English usually prefers the present perfect:

• has been

So the natural English translation is:

• The parsley has been in the planter box since Monday.

Hungarian does not need a special perfect form here. The sense of continuation up to the present comes from óta plus the present tense.

It can be used in more than one way.

In this sentence, it follows a time noun:

• hétfő óta = since Monday

But it can also appear in larger expressions, for example:

• azóta = since then
• amióta = since the time when / ever since

Examples:

• Azóta itt van. = He/She has been here since then.
• Amióta megérkezett, itt van. = He/She has been here since arriving.

So the pattern in your sentence is one very common use, but not the only one.

Not if you want the same meaning.

• hétfő óta = since Monday
• hétfőtől = from Monday / starting Monday

Hétfő óta clearly says the situation began on Monday and continues until now.

Hétfőtől often just marks a starting point and may sound more like scheduling or a change beginning then.

Compare:

• Hétfő óta az ültetőládában van a petrezselyem.
  = The parsley has been in the planter box since Monday.

• Hétfőtől az ültetőládában lesz a petrezselyem.
  = From Monday onward, the parsley will be in the planter box.

So óta is the right choice here.