Fx: A U.S. military transport aircraft with relief supplies is expected to land in Myanmar on Monday on a mission that officials hope will help forge a relationship to allow the United States to send in disaster experts.

Send in: To cause (someone) to arrive in or become involved in a particular place or situation.

According to the explaination, "send in" has a similar meaning with 'send". It seemed that "send" can also be placed as "send in" here.

So it is hard for me to decide how to use "send" and "send in", please help me. thank you!
Good question, b-c,

I think there's a sequence involved here. Everybody's been poised to go in, sort of like an invasion. The mission is on hold because of political considerations. "Send in" is sort of an idiom like "go in."

Shall we send them in now, or shall we wait awhile?

Send in the paratroopers vs. send the paratroopers. It's more dramatic with "in."

Under more normal circumstances, we'd say "Many countries are sending relief teams and disaster experts."

Prepositions lead to endless contradictory examples, but in general, I'd say you may choose to use "send in" when you're sending someone "into" some kind of a place, like a country or a house or an office.

Mr. Jones is here to see you, sir. (reply) Please send him in.

Best wishes, - A.
in tells the goal of the sending. It says where the experts will be sent. From the context we know that in means into Myanmar.
... to allow them to send in experts = ... to allow them to send experts into Myanmar.
Students: We have free audio pronunciation exercises.
Thank you very much, I'm very appreciated.
 CalifJim's reply was promoted to an answer.
Students: Are you brave enough to let our tutors analyse your pronunciation?