Bảng chữ cái ở bên dãy số.

Because Vietnamese usually does not use là for location.

• là is mainly used when one thing is identified as another:
  • Đây là bảng chữ cái. = This is the alphabet / alphabet chart.
• But for saying where something is, Vietnamese normally uses ở:
  • Bảng chữ cái ở bên dãy số. = The alphabet is beside the number sequence.

So using là here would sound unnatural.

Here, ở marks location. It is the key word that gives the meaning to be at / in / on / by.

So the pattern is:

• [thing] + ở + [place]

In this sentence:

• Bảng chữ cái = the alphabet / alphabet chart
• ở bên dãy số = is beside the number sequence

So ở is doing the job that English often expresses with is plus a place phrase.

Bảng chữ cái is the normal Vietnamese expression for alphabet.

Literally, it looks something like:

• bảng = board, chart, table
• chữ cái = letters

But as a set phrase, bảng chữ cái usually just means the alphabet.

Depending on context, especially in a classroom, it can also feel like the alphabet chart. So if this sentence is describing things on a wall, alphabet chart may be the most natural English interpretation.

Vietnamese often puts the main noun first and the describing part after it.

So:

• bảng chữ cái = literally something like board/chart of letters
• dãy số = sequence/row of numbers

This is a common Vietnamese pattern. English often does the opposite more often, with the describing word before the noun, but Vietnamese frequently goes noun + modifier.

Yes. bên means side or beside, depending on context.

In this sentence:

• ở bên dãy số means something like at the side of the number sequence or beside the number sequence

A very common alternative is bên cạnh, which more clearly means next to / beside:

• Bảng chữ cái ở bên cạnh dãy số.

That version may sound more explicit and natural to many learners.
So:

• bên = side / beside
• bên cạnh = next to / beside

Dãy số means a sequence, series, or row of numbers.

The best English translation depends on context:

• in math: number sequence
• on a classroom wall: row of numbers
• in some situations: number line might be used, though that is more specific

So dãy số is a flexible phrase. It refers to numbers arranged in order, but the exact English wording depends on what is physically or conceptually being described.

Vietnamese does not have articles like English a, an, and the.

That means a noun like bảng chữ cái can mean:

• the alphabet
• an alphabet chart
• the alphabet chart

Which one is correct depends on context.

So when translating from Vietnamese into English, you often have to add a or the based on the situation, even though there is no separate word for it in Vietnamese.

It is understandable, but many speakers might prefer a slightly fuller version such as:

• Bảng chữ cái ở bên cạnh dãy số.
• Bảng chữ cái ở cạnh dãy số.

These sound a bit more explicitly like The alphabet is next to the number sequence.

So the original sentence is not wrong, but bên cạnh may sound clearer and more natural if you want to emphasize physical position.