Mam dwie książki w torbie.

Yes. Mam means I have.

In Polish, the verb ending often tells you who the subject is, so you usually do not need to say ja for I.

• mam = I have
• masz = you have
• ma = he/she/it has

So Mam dwie książki w torbie is perfectly natural without ja.
If you said Ja mam dwie książki w torbie, that would add emphasis, like I have two books in the bag.

Polish does not have articles like English a/an and the.

So:

• książki can mean books
• w torbie can mean in a bag or in the bag

The exact meaning depends on context. This is very normal in Polish.

Because książka is a feminine noun.

The number two changes form depending on gender:

• dwa for many masculine and neuter nouns
• dwie for feminine nouns

So:

• dwa stoły = two tables
• dwa okna = two windows
• dwie książki = two books

Since książka is feminine, dwie is the correct form.

Because after two, Polish uses a plural noun.

So:

• jedna książka = one book
• dwie książki = two books

With 2, 3, 4, the noun usually takes the plural form that looks like the nominative/accusative plural for non-masculine-personal nouns.

Compare:

• dwie książki = two books
• trzy książki = three books
• cztery książki = four books

But after 5 and above, Polish usually changes to a different form:

• pięć książek = five books

That contrast is very important.

Yes. In this sentence, dwie książki is the direct object of mam, so the whole phrase is in the accusative.

What makes this confusing is that for feminine nouns like książka, the plural accusative form is the same as the plural nominative form:

• nominative plural: książki
• accusative plural: książki

So the function is accusative, even though the form looks identical to nominative.

Because the preposition w here means in in the sense of location, and with location it requires the locative case.

The base form is:

• torba = bag

The locative singular is:

• w torbie = in the bag / in a bag

So:

• torba = dictionary form
• w torbie = after w when something is located inside it

Not always.

With w, the case depends on the meaning:

• w + locative = being in something, location
  
  • w torbie = in the bag
• w + accusative = motion into something
  
  • wkładam książkę w torbę = I put the book into the bag

In your sentence, the books are already inside the bag, so Polish uses w torbie.

Yes. Polish word order is more flexible than English because case endings show grammatical roles.

The neutral version is:

• Mam dwie książki w torbie.

But you could also say:

• W torbie mam dwie książki.
  This emphasizes where they are.
• Dwie książki mam w torbie.
  This emphasizes two books.

All of these can be correct; the difference is mainly emphasis and style.

A rough English-friendly approximation is:

• dwie ≈ dvye
• książki ≈ kshyonsh-kee
• w torbie ≈ f TOR-byeh or v TOR-byeh depending on the sound around it

A few useful notes:

• ie often sounds like ye after a consonant
• ą is a nasal vowel; in książki it sounds roughly like on said through the nose
• w is pronounced like English v, but it can sound closer to f before a voiceless consonant

So the whole sentence is approximately:

Mahm dvye kshyonsh-kee f TOR-byeh.

That is only an approximation, but it is a useful starting point.

You would say:

Nie mam dwóch książek w torbie.

This is a very useful pattern because negation in Polish often changes the object from accusative to genitive.

Compare:

• Mam dwie książki w torbie.
• Nie mam dwóch książek w torbie.

Notice both changes:

• dwie → dwóch
• książki → książek

This is a common grammar point in Polish and often surprises English speakers.