んだけど is an expression that combines んだ (explaining/giving cause), with けど (but). Together they translate simply as ‘but...’, although the nuance is actually closer to ‘but with (A) as the reason… (B)’, while (B) is sometimes left unsaid. This implies that the speaker is looking for a reply, or does not want to say the (B) part of the sentence out of politeness/awkwardness.

• 助（たす）かるんだけど一人（ひとり）でできるから帰（かえ）ってもいいよ。
  I feel like you are being helpful, but I can handle it by myself, so you can go home.
• 私（わたし）は明日（あした）に釣（つ）りに行（い）くのですが先輩（せんぱい）も行（い）きませんか？
  I am going to go fishing tomorrow, but senpai, do you also want to go?
• 新（あたら）しいパソコンを買（か）いたいのだけど...どれを買（か）えばいいか分（わ）からない。
  I want to buy a new computer, but… I don't know which one to buy.

There are several different forms of this construction that are mostly interchangeable with each other. From most casual, to most formal, they are as follows:
んだけど、んだけれど、んだけれども、んだが、んですが
ん may be replaced with の in any of the above expressions. However, it will again make it slightly more formal.