חשבתי לקנות לה ספר בשוק, אבל אחרי זה בחרתי בתיק חדש.

חשבתי is the 1st person singular past form of לחשוב.

So grammatically it means I thought. But in a sentence like חשבתי לקנות..., it often means more than a simple thought:

• I thought of buying...
• I was thinking of buying...
• I intended to buy...

Hebrew past tense does not always separate these shades as clearly as English does, so context decides.

Because Hebrew commonly uses an infinitive after verbs like thought, wanted, decided, started, and so on.

• לקנות = to buy
• the ל־ here is part of the infinitive form

So:

• חשבתי לקנות = I thought of buying / I intended to buy

This is very normal Hebrew structure.

לה means to her or for her.

It is the preposition ל־ attached to the pronoun her. In this sentence, English would usually translate it more naturally as for her:

• לקנות לה ספר = to buy her a book

Hebrew often uses ל־ for the person who receives or benefits from something.

In Hebrew, when the indirect object is a short pronoun like לה, it very often comes before the direct object noun.

So the natural order is:

• לקנות לה ספר = to buy her a book

This is similar to English word order in buy her a book.
Putting לה later is much less natural here.

Hebrew normally has no indefinite article. There is no separate word for a/an.

So:

• ספר can mean a book
• תיק חדש can mean a new bag

Hebrew does have a definite article, ה־, for the.
So if it were definite, you would see forms like:

• הספר = the book
• התיק החדש = the new bag

The prefix ב־ usually means in, at, or sometimes with, depending on context.

So:

• שוק = market
• בשוק = in the market / at the market

Hebrew often attaches prepositions directly to the noun instead of writing them as separate words.

This is a very common question, because in normal unpointed Hebrew writing, those can look the same.

• בשוק can represent be-shuk = in a market
• or ba-shuk = in the market

Usually you know from:

• context
• pronunciation
• the meaning already given in the sentence

So this is one of those places where written Hebrew is a little less explicit than English.

אחרי זה means after that or afterwards.

Literally, it is something like after this/that.
Yes, it is very close in meaning to אחר כך.

Both are common, but:

• אחרי זה is very common in everyday speech
• אחר כך is also very common and can sound a bit more neutral

In this sentence, אחרי זה marks a change in what the speaker did next.

Because the verb לבחור often goes with ב־.

So:

• בחרתי בתיק חדש = I chose / opted for a new bag

This use of ב־ often gives the sense of choosing one option out of several possibilities.

You may also hear לבחור משהו without ב־, but לבחור ב־ is extremely common and very natural.

Here it is be-tik.

That is because the meaning is chose a new bag, not in the new bag. Also, the adjective helps you see that the phrase is indefinite:

• תיק חדש = a new bag
• התיק החדש = the new bag

If the phrase were definite, the adjective would also be definite. So בתיק חדש here is understood as be-tik chadash, not ba-tik.

In Hebrew, adjectives usually come after the noun, not before it.

So:

• תיק חדש = a new bag

Also, adjectives must agree with the noun in:

• gender
• number
• definiteness

תיק is masculine singular, so the adjective is חדש.

If the noun were feminine singular, you would expect חדשה instead.