Ten notatnik przydaje mi się w pracy.

Because notatnik is a masculine singular noun in the nominative case here.

The demonstrative ten must agree with the noun in gender, number, and case:

• ten = masculine singular
• ta = feminine singular
• to = neuter singular

So:

• ten notatnik = this notebook
• ta książka = this book
• to okno = this window

Since notatnik is masculine, ten is the correct form.

It is in the nominative singular.

In this sentence, notatnik is the thing being talked about as the grammatical subject:

• Ten notatnik przydaje mi się w pracy.

The verb przydaje się is in 3rd person singular, matching notatnik.

So the basic structure is:

• Ten notatnik = subject
• przydaje się = is useful / comes in handy
• mi = to me
• w pracy = at work / in work

Przydawać się / przydać się means something like:

• to be useful
• to come in handy
• to prove useful

In this sentence, przydaje się is the imperfective present form, so it suggests a general or repeated usefulness:

• Ten notatnik przydaje mi się w pracy. = This notebook is useful to me at work. = This notebook comes in handy for me at work.

A very literal translation would sound like This notebook proves useful to me at work, but in natural English we would usually say is useful or comes in handy.

They do different jobs.

• mi = to me
  It is the dative form of ja.

• się is part of the verb expression przydawać się / przydać się.
  Here it does not mean to oneself in the usual reflexive English sense. It is simply part of how this verb is used.

So:

• przydaje mi się = is useful to me / comes in handy for me

You should learn przydawać się as a whole pattern:

• coś przydaje się komuś
• something is useful to someone

Examples:

• Ta aplikacja przydaje mi się codziennie.
• Ten słownik przydaje się studentom.

Both mi and mnie can mean to me, but mi is the normal unstressed form and is much more natural in a neutral sentence like this.

So:

• Ten notatnik przydaje mi się w pracy. = neutral, natural
• Ten notatnik przydaje mnie się w pracy. = incorrect
• Ten notatnik przydaje się mnie w pracy. = also incorrect in normal Polish

If you wanted emphasis, Polish might use mnie, but the word order and structure would be different, for example in contrastive contexts. For this sentence, mi is the expected form.

Mi is in the dative case.

The verb pattern is:

• coś przydaje się komuś

So:

• coś = nominative, the useful thing
• komuś = dative, the person who benefits

In this sentence:

• Ten notatnik = nominative
• mi = dative

Other examples:

• Ten kurs przydaje się studentom.
  = This course is useful to students.

• Ta rada przyda ci się później.
  = This advice will be useful to you later.

W pracy means at work or in work-related situations.

The preposition w often takes the locative case when it means in or at a place/state.
The noun praca changes to pracy in the locative singular:

• nominative: praca
• locative: w pracy

So:

• w pracy = at work

This is very common Polish:

• Jestem w pracy. = I’m at work.
• Używam tego w pracy. = I use this at work.

The sentence is natural as written, but Polish word order is more flexible than English.

Neutral order:

• Ten notatnik przydaje mi się w pracy.

You could also hear:

• W pracy ten notatnik przydaje mi się.
• Mi się ten notatnik przydaje w pracy.
  though this sounds more marked or context-dependent

Changing the order usually changes the emphasis, not the core meaning.

For example:

• Ten notatnik przydaje mi się w pracy.
  neutral statement

• W pracy ten notatnik przydaje mi się.
  emphasis on at work

In beginner Polish, it is best to learn the neutral version first.

Because the subject is singular:

• Ten notatnik = this notebook

So the verb agrees with it:

• przydaje się = is useful / comes in handy for he/it

If the subject were plural, the verb would also be plural:

• Te notatniki przydają mi się w pracy.
  = These notebooks are useful to me at work.

So the verb form tells you that the subject is one notebook, not several.

This is a difference of aspect.

• przydawać się = imperfective
• przydać się = perfective

In the present tense, imperfective usually describes something general, habitual, or ongoing:

• Ten notatnik przydaje mi się w pracy.
  = This notebook is useful to me at work.

The perfective form is often used for the future:

• Ten notatnik przyda mi się w pracy.
  = This notebook will be useful to me at work.

So:

• przydaje się = is useful / comes in handy
• przyda się = will come in handy / will prove useful

Not exactly. It is understandable, but it is not the best match.

Przydaje się does not usually mean that the notebook actively helps in the same way a person helps. It means that it is useful, handy, or beneficial.

Better translations are:

• This notebook is useful to me at work.
• This notebook comes in handy at work.
• I find this notebook useful at work.

So helps me is possible as a rough interpretation, but is useful or comes in handy is more accurate.

Normally ten notatnik means this notebook.

Polish ten/ta/to is the basic demonstrative used for this. In real usage, it can sometimes overlap a little with English that, depending on context, gesture, or tone, but the default meaning for learners is:

• ten notatnik = this notebook

If you want to say that notebook more clearly, Polish often uses:

• tamten notatnik = that notebook over there / that one

So in this sentence, the safest interpretation is this notebook.