Добавь одну ложку мёда в чай.

Добавь is the informal singular imperative of the verb добавить.

So it means add! when speaking to one person you address with ты.

• добавить = to add
• Добавь! = add!

If you were speaking more formally or to several people, you would say Добавьте.

This sentence uses the perfective verb добавить, which is very natural for a single complete action: add one spoonful.

Because одна changes to одну in the accusative case.

The noun ложка is feminine, and in this sentence one spoonful is the direct object of the verb add, so both the numeral and the noun go into the accusative:

• nominative: одна ложка
• accusative: одну ложку

This is a very common pattern with feminine nouns.

For the same reason: ложка is the direct object, so it takes the accusative singular form.

• nominative: ложка = a spoon
• accusative: ложку

In Добавь одну ложку мёда, what is being added is one spoonful of honey, so ложку is in the accusative.

Because after a word of quantity like ложка in the sense of a spoonful of, Russian normally uses the genitive:

• ложка мёда = a spoonful of honey

So:

• мёд = nominative
• мёда = genitive singular

This is similar to patterns like:

• чашка чая = a cup of tea
• бутылка воды = a bottle of water
• кусок хлеба = a piece of bread

So here мёда answers the question ложку чего? = a spoonful of what?

Because в can be followed by different cases depending on the meaning.

Here, в чай means into the tea, so it uses the accusative:

• в чай = into the tea

If you were describing location, you would use the prepositional:

• в чае = in the tea

Compare:

• Положи сахар в чай. = Put sugar into the tea.
• Сахар уже в чае. = The sugar is already in the tea.

So in your sentence, the idea is movement/result: adding something into the tea.

In this sentence, ложка most naturally means a spoonful.

So одну ложку мёда is best understood as:

• one spoonful of honey

Russian often uses ложка this way when talking about ingredients or amounts.

Depending on context, it can literally mean a spoon, but here the quantity meaning is the natural one.

Yes. Russian word order is fairly flexible, and the basic meaning would stay the same.

For example:

• Добавь одну ложку мёда в чай.
• Добавь в чай одну ложку мёда.
• Одну ложку мёда добавь в чай.

These all mean roughly the same thing, but the emphasis changes a little.

• Добавь одну ложку мёда в чай sounds neutral.
• Добавь в чай одну ложку мёда puts into the tea a bit earlier.
• Одну ложку мёда at the front emphasizes the amount.

Yes, but the meaning changes slightly.

• Добавь = perfective imperative
  This is used for one complete action: add it.
• Добавляй = imperfective imperative
  This can sound like keep adding, add as a general practice, or go ahead and add depending on context.

In a recipe, instruction, or one-time command, Добавь is usually the most natural choice.

So:

• Добавь одну ложку мёда в чай. = Add one spoonful of honey to the tea.

Use Добавьте instead of Добавь:

• Добавьте одну ложку мёда в чай.

This can mean either:

• add ... when speaking politely to one person, or
• add ... when speaking to several people

Everything else in the sentence stays the same.

The main thing to notice is the letter ё.

• мёд and мёда are pronounced with yo, not plain e
• stress falls on ё, and ё is always stressed

So:

• мёд sounds roughly like myod
• мёда sounds roughly like MYO-da

In many texts, Russian writers omit the dots and write меда, but it is still understood as мёда here.

Not in this exact structure.

• ложка мёда = a spoonful of honey
• чай с мёдом = tea with honey

So these are different constructions:

• Добавь одну ложку мёда в чай. = Add one spoonful of honey to the tea.
• Это чай с мёдом. = This is tea with honey.

Use genitive after the quantity word ложка, but use с when you mean with.

They are:

• добавить = to add
• один = one
• ложка = spoon
• мёд = honey
• чай = tea

And the forms used in the sentence are:

• Добавь = imperative of добавить
• одну = accusative feminine of один
• ложку = accusative singular of ложка
• мёда = genitive singular of мёд
• чай in в чай = accusative singular after в meaning into