उसे फोन की लत है।

उसे means to him, to her, or to that person, depending on context.

Grammatically, it is the common short form of उसको, which comes from वह. In this sentence, Hindi uses a to someone ... is pattern, so the structure is literally something like:

To him/her, phone addiction is.

That is why उसे appears instead of a plain he/she form.

Because this sentence uses a Hindi pattern where the person experiencing a state is marked with को.

With लत होना and many similar expressions, Hindi does not usually say he/she is X in the same way English does. Instead, it says something closer to:

to him/her, X is

So:

• उसे फोन की लत है = He/She has a phone addiction
• literally = To him/her, phone addiction is

If you used वह, it would sound like you were building a different kind of sentence.

It can mean either him or her.

Hindi third-person pronouns do not normally distinguish he and she the way English does. So उसे can refer to:

• him
• her
• sometimes that person

In this sentence alone, you cannot tell the person’s gender.

Because का / की / के agrees with the noun that follows it, and here that noun is लत.

लत is a feminine singular noun, so the correct form is की:

• फोन की लत

This is true even though फोन is not feminine.

A useful comparison:

• फोन का इस्तेमाल = use of the phone
  because इस्तेमाल is masculine singular
• फोन की लत = addiction to the phone
  because लत is feminine singular
• फोन के फायदे = advantages of the phone
  because फायदे is plural

Word-for-word, it can look that way at first, but that is not how it is understood.

In Hindi, X की लत is a normal pattern meaning addiction to X. So:

• शराब की लत = addiction to alcohol
• चाय की लत = addiction to tea
• फोन की लत = addiction to the phone / phone addiction

So की here does not mean true ownership. It is just part of the standard Hindi way of expressing this idea.

No, फोन से लत would not be the normal idiomatic way to say it.

English uses addicted to, but Hindi usually uses a different structure:

• फोन की लत है
• मोबाइल की लत है

So it is best not to translate the English preposition to directly.

Another possible Hindi pattern is:

• वह फोन का आदी है for a male
• वह phone की आदी है for a female

But for your sentence, फोन की लत है is the most straightforward pattern to learn.

This is an important difference.

• आदत = habit
• लत = addiction

आदत can be neutral, positive, or negative:

• मुझे सुबह जल्दी उठने की आदत है = I have a habit of waking up early

लत is stronger and usually negative:

• उसे फोन की लत है = He/She is addicted to the phone

So if you say फोन की आदत, it sounds weaker than फोन की लत.

Yes. उसे and उसको both work here.

So these mean the same thing:

• उसे फोन की लत है
• उसको फोन की लत है

उसे is simply a shorter, very common form. In everyday Hindi, both are natural.

Hindi often does not use a separate verb that works exactly like English have.

Here, है literally means is, but the whole pattern उसे ... है expresses something like he/she has ... or he/she experiences ....

So:

• उसे फोन की लत है = literally To him/her, phone addiction is
• natural English = He/She has a phone addiction

This same pattern is common with feelings and conditions too:

• उसे बुखार है = He/She has a fever
• मुझे डर है = I am afraid / I have fear
• उसे आदत है = He/She is used to it / has a habit

In normal usage, it usually means being addicted to using the phone, not being emotionally attached to one physical device.

So the idea is usually:

• always checking the phone
• constantly using it
• being glued to it

If you want to make that extra explicit, Hindi could say:

• उसे फोन इस्तेमाल करने की लत है
• उसे मोबाइल चलाने की लत है

But फोन की लत already sounds natural and is usually understood correctly.

Often, yes.

In modern everyday Hindi, if you mean mobile phone / smartphone addiction, many speakers would naturally say:

• उसे मोबाइल की लत है

That said, फोन की लत is also perfectly understandable and natural. It just sounds a little more general.

So:

• फोन = phone in general
• मोबाइल = mobile phone, often more specific in modern conversation

The given order is the most normal and neutral one:

• उसे फोन की लत है

Hindi word order is somewhat flexible, but changing it usually adds emphasis or sounds marked. For example:

• फोन की लत उसे है = possible, but more contrastive or emphatic
• उसे है फोन की लत = very marked, dramatic, or poetic

So for ordinary speech and writing, the original order is the best one to use.