2つのものが何かの点で同じくらいだと言うときは、ようすを表すことばを2つの as で挟みます（as + 単語 + as）。単語は -er をつけず、そのままの形にしておきます。
as + 単語 + as
Amina is as tall as Lars.
This book is as interesting as that one.
Hiroshi runs as fast as his brother.
My bag is as heavy as yours.
Not "Amina is as taller as Lars." — say "Amina is as tall as Lars.". as ... as の間では、単語は変化させずにそのままにします（"as taller as" ではなく as tall as）。
やってみよう: Hiroshi runs ___ his brother. (as fast as / as faster as / as fast than) 答え: as fast as — 間に挟む単語はそのままの形にします（"as faster as" ではなく as fast as）。そして両側に as を置くので、うしろも than ではなく as になります（as fast as his brother）。
not as ... as
一方がもう一方ほど〜ではないと言うときは、not を足して not as + 単語 + as にします。
She is not as tall as her sister. (= shorter)
Today isn't as cold as yesterday. (= warmer)
This café isn't as busy as the one downtown.
やってみよう: Today isn't ___ yesterday. It's warmer. (as cold as / as colder as / as cold than) 答え: as cold as — as ... as の形はそのままで、not（"isn't"）を加えるだけです（isn't as cold as yesterday）。単語はそのままの形にして（"colder" は不可）、両側とも as なので、2つ目も "than" ではなく as になります。
as ... as？ それとも -er than？
選ぶ前に、同じくらいなのか、一方が上回っているのかを考えましょう。同じなら as + そのままの単語 + as。一方が上なら -er / more ... than。一方が下なら not as ... as を使います。
The same: Amina is as tall as Lars. (they measure the same)
One is more: Amina is taller than Lars.
One is less: Amina is not as tall as Lars. (= she is shorter)
Watch the word: it stays plain inside as ... as (as tall as), but takes -er with than (taller than).
Not "Amina is as tall than Lars." — say "Amina is as tall as Lars. / Amina is taller than Lars.". as は as とセット、-er は than とセットで使います。2つを混ぜてはいけません。
やってみよう: Mei も Sofia も身長はちょうど 1.65 メートルです。どちらかのほうが背が高いということはありません。 同じ数値の2つのものに当てはまるのはどちらですか？ (Mei is as tall as Sofia. / Mei is taller than Sofia.) 答え: Mei is as tall as Sofia. — 2人は同じ身長なので、as + そのままの単語 + as を使って as tall as にします。"Taller than" だとどちらかがより高いことになってしまいますが、実際はそうではありません。
やってみよう: My phone is ___ yours, but it isn't ___ that new one. (as new as … as expensive as / as newer as … as expensive than / as new than … as expensive as) 答え: as new as … as expensive as — 両方の空欄で同じパターン（as + そのままの単語 + as）を使います。したがって as new as と as expensive as になります。肯定文でも "isn't" のうしろでも、単語はそのままの形を保ち、両側に as を置きます。