אין ספק.

It is usually pronounced ein saFEK.

• אין = ein
• ספק here = saFEK (stress on the last syllable)

A rough English approximation for ein is the vowel sound in rain, but shorter.

Because Hebrew often does not use a separate word for is/are in the present tense.

So אין ספק is literally something like:

• אין = there is not / there isn’t
• ספק = doubt

So the whole sentence is literally There is no doubt.

Here, אין is the standard Hebrew word used to say that something does not exist or is not present.

In other words, it means:

• there is no...
• there isn’t...
• sometimes there are no...

That is why אין ספק means there is no doubt.

Because אין and לא do different jobs.

• לא usually negates a verb: I do not go, he did not say
• אין negates existence: there is no..., there isn’t...

So:

• אין ספק = correct
• לא ספק = not natural Hebrew

A good rule: if English has there is no / there isn’t, Hebrew often uses אין.

In this sentence, ספק means doubt or uncertainty.

But this is a useful spelling warning: Hebrew ספק can also be a different word meaning supplier.

Without vowel marks, both are written the same way:

• סָפֵק = doubt → pronounced saFEK
• סַפָּק = supplier → pronounced saPAK

In אין ספק, the meaning is definitely doubt, not supplier.

Yes. It can absolutely stand alone as a full sentence.

It works like a short statement meaning:

• No doubt.
• There’s no doubt.
• Certainly.

It is also very common at the beginning of a longer sentence, especially with ש־:

• אין ספק שהוא צודק.
• There is no doubt that he is right.

Use אין ספק ש־...

The ש־ means that.

For example:

• אין ספק שהיא מוכשרת.
  There is no doubt that she is talented.

• אין ספק שזה נכון.
  There is no doubt that this is true.

In everyday Hebrew, this pattern is extremely common.

It is fairly neutral and very common. It can sound:

• natural in speech
• normal in writing
• slightly strong or confident in tone

It is not slang, and it is not overly literary. It is a very standard phrase.

Yes. Common stronger versions include:

• אין שום ספק = there is absolutely no doubt
• אין כל ספק = a slightly more formal emphatic version
• בלי ספק / ללא ספק = without doubt / undoubtedly

These are all natural, but אין ספק is the basic and most straightforward form.

Because the phrase is talking about doubt in general, not a specific doubt.

So Hebrew uses the bare noun:

• אין ספק = there is no doubt

Using הספק here would sound unnatural in this expression.

If you want stronger emphasis, Hebrew normally adds words like:

• אין שום ספק
• אין כל ספק

rather than making ספק definite.