やってみよう: I ___ that film three times. (have seen / have see / have saw) 答え: have seen — have の後ろには過去分詞を使うので have seen となります。「see」は動詞の原形で、「saw」は単独で使う過去形です。have と組み合わせるときは seen になります。
he, she, it には has
I, you, we, they のときは have を使います。主語が he, she, it のときは has に変えましょう。
I have finished. / You have finished.
She has finished. / He has finished.
We have met them. / It has stopped raining.
Not "She have finished her homework." — say "She has finished her homework.". 主語が he, she, it のときは have ではなく has を使います。
やってみよう: She ___ her homework already. (has finished / have finished / has finish) 答え: has finished — 主語が she なので have ではなく has を使い、she has finished とします。has の後ろは動詞の原形 finish ではなく過去分詞 finished になります。
She has lived in Haffenville since May. (live → lived)
We have known each other for years. (know → known)
He has been ill since Monday. (be → been)
Not "We have know each other since school." — say "We have known each other since school.". have / has の後ろでは動詞の原形 know ではなく、過去分詞の known を使います。
やってみよう: We have ___ each other for ten years. (know) (known / knowed / knew) 答え: known — know の過去分詞は不規則変化の known です（knowed という形はありません）。knew は単独で使う過去形なので、have/has の後ろには過去分詞の have known を使います。
haven't / hasn't
否定文にするときは、haven't または hasn't + 過去分詞を使います（haven't = have not、hasn't = has not）。
I haven't seen that film.
He hasn't finished yet.
We haven't been to Senfarren.
やってみよう: He ___ his lunch yet. (hasn't eaten / haven't eaten / hasn't ate) 答え: hasn't eaten — 主語が he なので否定形は haven't ではなく hasn't になります。後ろには過去形 ate ではなく過去分詞 eaten を使って he hasn't eaten とします。
Have you ever ...?
疑問文にするには、文頭に Have または Has を出します。これまでの経験をたずねるときは ever を加えて、Have you ever ...? と言います。
Have you ever eaten sushi?
Has she ever been to Senfarren?
Have they finished?
やってみよう: ___ you ever been to Senfarren? (Have / Do / Are) 答え: Have — これまでの経験をたずねるときは Have で始めて Have you ever been...? とします。「Do you been」や「Are you been」という形はなく、Have + 過去分詞を使います。
これまでの「経験」を表す
have / has + 過去分詞の大切な使い道の 1 つが、人生における経験を話すことです。具体的な時期を言わずに「これまでに〜したことがある」と伝えるときに使います。よく使われる単語：ever（疑問文で「今までに」）、never（一度も〜ない）、before（以前に）。
I have visited Senfarren. (some time in my life, I don't say when)
She has never eaten sushi.
Have you ever ridden a horse before?
やってみよう: You want to ask about your friend's whole life, not one special day. Which fits? (Have you ever been to Senfarren? / Did you go to Senfarren yesterday? / Are you going to Senfarren?) 答え: Have you ever been to Senfarren? — 時期を特定せず今までの経験をたずねるときは、have + 過去分詞に ever を添えて Have you ever been...? と言います。「Did you go... yesterday」は特定の日をたずねる過去形、「Are you going」は未来を表す文です。
たった今起こったことを表す
have / has + 過去分詞は、少し前に完了した出来事で、その結果が今に関係しているときにも使います。ここでは just や already がよく一緒に使われます。
I have just finished my work. (so I'm free now)
She has already eaten. (so she isn't hungry now)
We have lost our keys. (so we can't get in now)
やってみよう: Which sentence shows something that finished a moment ago, with a result now? (I have just missed the bus. / I miss the bus every day. / I will miss the bus.) 答え: I have just missed the bus. — 「Have just missed」は直前に乗り遅れた結果、今待たなければならない状態を表します。「Miss every day」は日頃の習慣、「will miss」はこれからの未来のことです。
過去分詞の前に置く just と already
just は「たった今」、already は「もう／すでに」という意味です。どちらも have / has と動詞のあいだ、つまり過去分詞の直前に置きます。
I have just finished.
She has already left.
We have already seen it.
やってみよう: Where does "already" go? Pick the right sentence. (She has already left. / She already left. / She already has left.) 答え: She has already left. — already は has と過去分詞のあいだに入るので、She has already left となります。「She already left」は has が抜けており、「She already has left」は already の位置が前すぎます。
文末に置く yet
yet は「まだ（〜していない）／もう（〜したか）」という意味で、否定文や疑問文の文末に置きます。
Have you eaten yet?
They haven't arrived yet.
I haven't finished yet.
やってみよう: He's still at work. He ___. (hasn't finished yet / doesn't finished yet / yet hasn't finished) 答え: hasn't finished yet — 否定文では hasn't + 過去分詞を使い、文末に yet を置いて He hasn't finished yet とします。「Doesn't finished」は形が混ざっており、「yet hasn't finished」は yet が文頭にきてしまっています。