बहन ने मेरा फोन मेज़ के ऊपर रखा।

ने marks the subject in many completed transitive actions in Hindi.

So in बहन ने ... रखा, the action put/placed is completed, and रखना is being used transitively, so the doer बहन takes ने.

It does not translate directly into English. It is a grammar marker, not a word like by or did.

Because in this kind of ने sentence, the verb usually agrees with the direct object, not the subject.

Here the direct object is फोन, which is normally treated as masculine singular in Hindi. So the verb is रखा.

Compare:

• बहन ने मेरी किताब मेज़ पर रखी।
  Here किताब is feminine, so the verb becomes रखी.

So the verb is matching फोन, not बहन.

Because मेरा / मेरी / मेरे agrees with the thing possessed, not with the owner.

Here the possessed thing is फोन, which is masculine singular, so you use मेरा:

• मेरा फोन = my phone
• मेरी किताब = my book
• मेरे जूते = my shoes

So मेरा is chosen because of फोन, not because of the speaker.

Because फोन is an inanimate direct object, and in Hindi those often appear without को.

Hindi commonly uses को with:

• people
• animate beings
• sometimes very specific or emphasized objects

But with everyday inanimate objects, leaving out को is very normal:

• मैंने फोन रखा।
• उसने किताब उठाई।

You can sometimes use को for extra emphasis or specificity, but it is not the usual choice here.

मेज़ के ऊपर literally means above/on top of the table, and in normal use it often just means on the table.

के ऊपर is a compound postposition. Hindi often uses structures like:

• के ऊपर = on top of / above
• के नीचे = under / below
• के पास = near / with
• के बाद = after

So here के is part of the larger postpositional phrase के ऊपर.

Because nouns before postpositions go into the oblique case, but some nouns do not visibly change in the singular.

मेज़ is one of those. So:

• direct: मेज़
• oblique: मेज़

There is no visible difference here.

Compare a noun that does change:

• लड़का → लड़के के ऊपर
• कमरा → कमरे में

So the case is changing grammatically, but मेज़ happens to look the same.

Not necessarily by itself. बहन literally means sister.

In context, Hindi often leaves out possessives with family words when the meaning is obvious. So depending on the situation, बहन could mean:

• my sister
• his/her sister
• the sister

If you want to make it explicit, you can say:

• मेरी बहन ने मेरा फोन मेज़ के ऊपर रखा।

Yes. मेज़ पर is very common and often the most natural way to say on the table.

So these are both possible:

• बहन ने मेरा फोन मेज़ पर रखा।
• बहन ने मेरा फोन मेज़ के ऊपर रखा।

The difference is small:

• पर = on
• के ऊपर = on top of / above

In many everyday situations, पर sounds simpler and more natural.

In this sentence, रखा already shows a completed action. Hindi does not need a separate word like did or was here.

So:

• बहन ने ... रखा। = sister put/placed ...

If you add था, the meaning changes a bit:

• बहन ने मेरा phone मेज़ पर रखा था।
  = sister had put / had placed my phone on the table

So the sentence without था is a normal simple completed past.

The basic Hindi order is:

Subject + Object + Location + Verb

So this sentence follows a very normal pattern:

• बहन ने = subject
• मेरा फोन = object
• मेज़ के ऊपर = location
• रखा = verb

Hindi word order is somewhat flexible, though. You can move parts around for emphasis, but the verb usually stays near the end.

For example:

• बहन ने मेरा फोन मेज़ पर रखा।
  neutral
• मेरा फोन बहन ने मेज़ पर रखा।
  emphasizes my phone
• मेज़ पर बहन ने मेरा फोन रखा।
  emphasizes on the table

For a learner, the original order is a good default.