Learn Hardcore Japanese: tomodati ha hanabi no syasin wo kaigai ni iru kazoku he okutta. - My friend sent photos of the fireworks to family overseas.

The particle の here is linking two nouns, functioning like the English “of.” 花火の写真 literally means “fireworks’ photo,” or more naturally “photos of fireworks.”

Why is は used with 友達は instead of が?

は marks the topic rather than the subject. By saying 友達は, you’re setting “my friend” as the topic of the sentence (i.e. “As for my friend…”). If you used が (友達が送った), the nuance would shift to introducing “the friend” as new information or emphasizing that it was specifically the friend who sent them.

Why do we use 家族へ送った instead of 家族に送った? Aren’t へ and に both used for “to”?

Both particles can indicate direction or recipient, but they have slightly different feels.

家族に送った is neutral: “sent to the family.”
家族へ送った has a more directional nuance, like “sent off toward the family.”
In many contexts they’re interchangeable, but へ often feels a bit more “to that direction/recipient.”

What is 海外にいる家族 doing in this sentence? It seems to be three elements in a row.

This is a relative clause. 海外にいる (“who are overseas”) modifies 家族 (“family”) without any extra particle. So 海外にいる家族 means “(the) family who are overseas.” In English we’d say “my family overseas” or “the family that’s overseas.”

Why is いる used instead of ある for 海外にいる家族?

In Japanese, いる is used for animate existence (people or animals), while ある is for inanimate objects. Since 家族 are people, you use いる to say they “are located” overseas.

There are no plural markers on 写真 or 家族. How do we know it means “photos” (plural) and “family” (collective)?

Japanese generally does not mark plurals the way English does. Context tells you quantity. Here, because your friend sent multiple snapshots, you naturally interpret 写真 as plural. 家族 likewise refers to the family collectively—English doesn’t need an -s to understand that.

Why is the verb 送った in plain past form instead of a polite form like 送りました?

The sentence as given is in casual/plain style. In a conversation with friends or in writing a diary, you’d typically use 送った. If you wanted to be more polite (e.g., in a letter or formal report), you could say 送りました.

Can the word order change? For example, could I say 花火の写真を海外にいる家族へ友達は送った?

Technically yes: Japanese is relatively flexible because particles mark each word’s role. However, the original order (友達は花火の写真を … 送った) is the most natural: topic → direct object → indirect object → verb. Putting 友達は at the very end sounds odd if you’re trying to keep “my friend” as the topic.