nagai zikan sumaho wo miru tabi ni, kubi ya kosi ga tukaremasu.

Because 長い is an i-adjective directly modifying the noun 時間.

• 長い時間 = a long time / a long period of time
• 長く is the adverb form, so it modifies verbs instead

For example:

• 長い時間スマホを見る = to look at a smartphone for a long time
• 長く見る = to look for a long time

So in this sentence, 長い is correct because it is attached to 時間.

In 長い時間スマホを見る, the phrase 長い時間 works as a time-duration expression. Expressions of duration often appear without a particle in Japanese.

So:

• 長い時間スマホを見る = look at a smartphone for a long time

This is similar to other duration expressions:

• 一時間勉強する = study for one hour
• 毎日運動する = exercise every day

You can sometimes see particles like に with points in time, but duration phrases like 長い時間 often stand alone.

たびに means whenever, every time, or each time.

So:

• スマホを見るたびに = every time I look at my smartphone
• 長い時間スマホを見るたびに = whenever I look at my smartphone for a long time

A very important grammar point:

• dictionary form + たびに
• noun + のたびに

Examples:

• 行くたびに = every time I go
• 会うたびに = every time I meet
• 旅行のたびに = every time I travel

In your sentence, 見るたびに shows something that happens repeatedly as a result of the action.

Because たびに normally attaches to the dictionary form of a verb, not the past form.

So the pattern is:

• verb dictionary form + たびに

Examples:

• 読むたびに
• 聞くたびに
• 見るたびに

Even though English often uses every time I look / whenever I look, Japanese still uses the dictionary form here.

Because スマホ is the direct object of 見る.

• スマホを見る = to look at a smartphone

The verb 見る takes を for the thing being looked at.

So:

• 映画を見る = watch a movie
• 空を見る = look at the sky
• スマホを見る = look at a smartphone

Even though English says look at, Japanese just uses object + を + 見る.

や is used to list examples in a non-exhaustive way.

So:

• 首や腰 = the neck and lower back, among possibly other body parts
• 首と腰 = the neck and the lower back as a complete, exact list

Using や sounds a little more natural here because the speaker is mentioning typical areas that get tired, without insisting those are the only ones.

Because 疲れる is an intransitive verb. It means something like to get tired or to become fatigued.

That means the body parts experiencing the condition are marked with が:

• 首が疲れる = the neck gets tired
• 腰が疲れる = the lower back gets tired

You do not say 首を疲れる.

This is different from English, where we might think of I tire my neck, but Japanese usually expresses it as the neck itself becoming tired.

Yes. In Japanese, 疲れる can be used for the whole person or for body parts.

Examples:

• 足が疲れた = my legs got tired
• 目が疲れる = my eyes get tired
• 首が疲れる = my neck gets tired

This is very natural Japanese. Depending on context, English might translate it as:

• my neck gets tired
• my neck feels strained
• my neck gets fatigued

So even if English sometimes prefers hurts or gets sore, 疲れる is normal in Japanese.

In dictionary definitions, 腰 can be translated as waist, hips, or lower back, but in this kind of sentence it usually means lower back.

So here:

• 首や腰が疲れます most naturally means my neck and lower back get tired

If you translate 腰 as waist here, it may sound a little unnatural in English.

Because Japanese often leaves out the subject when it is obvious from context.

In this sentence, the meaning is naturally understood as something like:

• When I look at my smartphone for a long time, my neck and lower back get tired.

Japanese does this all the time. If the speaker added 私は, it would still be grammatical, but it is often unnecessary unless there is contrast or emphasis.

In Japanese, the non-past form often expresses:

• present actions
• future actions
• habits
• general truths

So 疲れます here means something like:

• get tired
• tend to get tired
• become tired

It is describing a repeated result:

• Whenever I look at my smartphone for a long time, my neck and lower back get tired.

So this is a normal use of the non-past form.

Yes, but the nuance changes.

• 疲れます = get tired / fatigued / strained
• 痛くなります = start to hurt / become painful

So:

• 首や腰が疲れます suggests fatigue or strain
• 首や腰が痛くなります suggests actual pain

If the speaker wants to say the neck and lower back become sore or painful, 痛くなります may be better. If the idea is more general physical fatigue from posture, 疲れます works well.

Yes. スマホ is a very common shortened form of スマートフォン and is completely natural in everyday Japanese.

• スマホ = casual, common, natural in conversation
• スマートフォン = more formal or full-length

In most everyday contexts, スマホを見る sounds more natural than スマートフォンを見る.