चाबी ताले के पास है; टिफ़िन बैग में है।

Because ताला changes to its oblique form before a postposition.
In Hindi, nouns often change form when followed by words like में, पर, के पास, से, etc.

• Direct form: ताला
• Oblique form: ताले

So ताले के पास means near the lock.

के पास usually means near, close to, or sometimes with/in the possession of, depending on context.

In this sentence:

• चाबी ताले के पास है = The key is near the lock

In another sentence, it could mean possession:

• मेरे पास चाबी है = I have a key

So पास is very common and useful, but its exact English translation depends on context.

में is the postposition meaning in or inside.

So:

• बैग में = in the bag

This is the normal way Hindi expresses location inside something.

Not all nouns visibly change in the oblique form.
बैग is a borrowed word, and nouns like this often look the same in both direct and oblique forms.

So:

• Direct: बैग
• Oblique: बैग

Even though the form does not change, it is still functioning before the postposition में.

है is the present-tense form of to be for a singular subject.

Here:

• चाबी ... है = The key is ...
• टिफ़िन ... है = The tiffin/lunchbox is ...

Since both subjects are singular, है is correct in both clauses.
If they were plural, Hindi would usually use हैं.

No—not here.
In the present tense, है does not change for masculine vs. feminine singular.

So both of these are correct:

• चाबी यहाँ है = The key is here
• ताला यहाँ है = The lock is here

Gender matters in many parts of Hindi grammar, but not in this particular use of है.

Hindi normally does not use articles like English a/an/the.

So:

• चाबी can mean a key or the key
• ताला can mean a lock or the lock
• बैग can mean a bag or the bag

The exact meaning depends on context.

Yes, Hindi word order is somewhat flexible, especially in simple location sentences.

These are both natural:

• चाबी ताले के पास है
• ताले ke पास चाबी है

Both mean essentially the same thing: The key is near the lock.
The first version starts with the thing being talked about (चाबी), while the second starts with the location.

In Hindi, these are called postpositions, not prepositions.

English uses prepositions before the noun:

• in the bag
• near the lock

Hindi puts the equivalent words after the noun:

• बैग में
• ताले के पास

That is why learners often say Hindi has postpositions instead of prepositions.

टिफ़िन is a very common word in Indian usage. Depending on context, it can mean:

• a lunchbox
• a packed meal
• sometimes just a light meal/snack

In टिफ़िन बैग में है, it most naturally means the lunchbox/tiffin is in the bag.

That dot is called a nukta. It is used to represent sounds that entered Hindi through Persian, Arabic, English, and other languages.

• फ is traditionally closer to an aspirated ph
• फ़ represents f

So टिफ़िन is spelled with फ़ to match the f sound in tiffin.

The semicolon simply separates two closely related statements:

• चाबी ताले के पास है
• टिफ़िन बैग में है

It works much like English punctuation here.
A full stop or even a comma might also appear in informal writing, but the semicolon neatly links the two short location statements.