במבצע הזה כל מוצר שני זול יותר, ולכן קניתי שני בקבוקי מים.

במבצע הזה means in this sale / in this promotion.

It is made of:

• ב־ = in / during
• מבצע = sale, special offer, promotion
• הזה = this

So literally it is something like in this promotion.

In Hebrew, it is very common to use ב־ with time periods or situations:

• ביום הזה = on this day
• בשיעור הזה = in this lesson
• במבצע הזה = in this sale

Here it means as part of this special offer.

In Hebrew, demonstratives like הזה / הזאת / האלה usually come after the noun, not before it.

So:

• המבצע הזה = this sale
• הספר הזה = this book
• הילדה הזאת = this girl

That is different from English, where this comes first.

In your sentence, the noun appears with the preposition ב־, so:

• המבצע הזה = this sale
• במבצע הזה = in this sale

Also notice that when a noun is definite with this, Hebrew normally uses ה־ on the noun as well, even if a preposition is attached:

• ב + ה + מבצע הזה → במבצע הזה

כל מוצר שני means every second product or every other product.

This can feel strange if you translate word by word:

• כל = every
• מוצר = product
• שני = second

But together, כל X שני is a common Hebrew pattern meaning:

• every second X
• every other X

Examples:

• כל יום שני = every second day / every other day
• כל בית שני = every second house
• כל מוצר שני = every second product

So in the context of a sale, it means that when you buy items, every second one is cheaper.

Because this is not talking about the second product as one specific item. It is using the pattern כל + singular noun + ordinal to mean every second ...

Compare:

• המוצר השני = the second product
  This refers to one specific product in a sequence.
• כל מוצר שני = every second product / every other product
  This is a general repeating pattern.

Also, after כל, the noun is often singular and usually not made definite with ה־:

• כל ילד = every child
• כל יום = every day
• כל מוצר שני = every second product

Here שני is an ordinal number, meaning second.

Hebrew distinguishes between:

• cardinal numbers: one, two, three...
• ordinal numbers: first, second, third...

So:

• שניים = two (masculine cardinal)
• שני = two before a masculine noun, or second depending on context
• שני in מוצר שני = second

That can be confusing, because the same form שני appears in different jobs.

In this sentence:

• מוצר שני = second product
• שני בקבוקי מים = two bottles of water

So the first שני is ordinal (second), while the second שני is the number two before a masculine plural noun.

זול יותר means cheaper.

Literally:

• זול = cheap
• יותר = more

So Hebrew often forms the comparative with:

adjective + יותר

Examples:

• גדול יותר = bigger
• מהיר יותר = faster
• זול יותר = cheaper

Unlike English, Hebrew does not usually add something like -er to the adjective. It uses יותר instead.

If you want to say cheaper than..., Hebrew often uses מ־:

• זול יותר מהמוצר הראשון = cheaper than the first product

In your sentence, the than part is not stated, because the sale context already makes it clear: the second item is cheaper than the regular one or than the first item.

ולכן means and therefore, so, or and that’s why.

It is made of:

• ו־ = and
• לכן = therefore / so

Together:

• ולכן = and therefore / so

This is a very natural connector in Hebrew when one clause is the result of the previous one:

• היה מבצע, ולכן קניתי יותר.
  There was a sale, so I bought more.

In everyday English, you might translate it simply as so.

Because Hebrew verbs usually already show the subject.

קניתי means I bought:

• the verb root is connected to buy
• the ending ־תי marks I in the past tense

So Hebrew often drops the subject pronoun when it is obvious from the verb:

• קניתי = I bought
• הלכתי = I went
• אמרתי = I said

You can say אני קניתי, but that adds emphasis, something like:

• I bought it
• I was the one who bought it

Without emphasis, קניתי is the normal choice.

Because Hebrew uses a special form of two before nouns.

For masculine nouns:

• שניים = two when standing alone
• שני = two before a masculine noun

For feminine nouns:

• שתיים = two when standing alone
• שתי = two before a feminine noun

So:

• שניים = two
• שני בקבוקים / שני בקבוקי מים = two bottles / two bottles of water

Since בקבוק is masculine, שני is the correct form before it.

Compare:

• שני ספרים = two books
• שתי מכוניות = two cars

Because this is a construct chain in Hebrew.

בקבוקי מים literally means bottles of water.

The first noun goes into the construct form:

• singular: בקבוק = bottle
• plural absolute: בקבוקים = bottles
• plural construct: בקבוקי = bottles of ...

So:

• שני בקבוקים = two bottles
• שני בקבוקי מים = two bottles of water

This is one of the most common Hebrew structures:

• כוס קפה = cup of coffee
• בקבוק יין = bottle of wine
• שלוש כוסות מים = three cups of water

Hebrew often prefers this structure over using של.

Yes, you could, and people would understand it. It means the same thing: two bottles of water.

But שני בקבוקי מים is usually more natural and compact in standard Hebrew.

Compare:

• שני בקבוקי מים = more concise, very natural
• שני בקבוקים של מים = also possible, a bit more expanded

Hebrew uses both patterns, but the construct form is especially common with fixed combinations like this.

מים is the normal Hebrew word for water, and it looks plural in form.

Historically and grammatically, it is a plural form, but in meaning it functions as the ordinary word for water. So learners should mostly just memorize it as the standard noun.

That is why you say:

• בקבוק מים = a bottle of water
• שתיתי מים = I drank water

Even though English uses a singular-looking mass noun, Hebrew uses מים.

This is similar to a few other Hebrew nouns that have unusual number behavior. For practical purposes, just treat מים as the normal word for water.

Because Hebrew normally uses כל + singular noun for every.

So:

• כל יום = every day
• כל ילד = every child
• כל מוצר = every product

If you use a plural after כל, the meaning often shifts more toward all rather than every:

• כל המוצרים = all the products

So in your sentence:

• כל מוצר שני = every second product not
• all second products

That singular after כל is completely normal Hebrew grammar.