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When using the complement 到, like in 买不到, being able/unable to achieve the goal - is connected to the subject or object? For example 这本新书很受欢迎， 你现在去买， 一定买不到。 (cannot buy because there are no books left..) Will it be correct to say:
我忘了信用卡， 买不到礼物 or 今天我很忙， 买不到礼物 Here the ability/inability is connected to 我
＂实用现代汉语语法＂distinguishes 3 types of 可能补语；A、B 与 C 类如下：A：在动词结果补语或趋向补语之间插入＂得＂或＂不＂可以构成 A 类补语。＂吃饱＂－－－＂吃得饱＂、＂吃不饱＂（买到－－－买得到、买不到）。B：B类可能补语是由＂得／不＋了＂构成的。C：动词和形容词后可以只用＂得／不得＂作补语，叫 C 类可能补语。
Q and comment #2 (着zhao2 作结果补语，表示动作达到了目的，多用于口语） have type A, comment #1 has type B。
＂实用现代汉语语法＂：B 类可能补语于 A 类的区别：
虽然在表达＂主、客观条件是否容许实现＂这一点上 A 类可能补语与 B 类有相同之处，但 A 类总与动作的结果或趋向联系，而 B 类则与结果或趋向无关。试比较：
１）今天晚上我有事，看不完这本书了。（A 类，不能实现＂看完＂）
２）今天晚上我有事，看不了这本书了。（B 类，不能实现＂看＂）（supporting comment #1)
包含 B 类可能补语的句子的结构特点
包含 B 类可能补语的句子，结构特点基本与包含 A 类可能补语的句子相同，值得提出的有以下两点：
１。动词形容词与 B 类结合要比 A 类来得容易，结合面也宽些，连某些动补式动词（如＂扩大、延长，埋没，提高＂等）、前面有状语的动词甚至有其他补语的动词都可以带 B 类补语。例如：
１）我们这里埋没不了人才。
２）他这个人，早来不了。
３）这些土地是我们的，谁也抢不了去！
这是因为 B 类可能补语主要用于口语，所以只有口语中不常用的动词，如＂逾、著、恭候、寻（找）。。。＂、表示使令义的＂使、让、叫＂以及能愿动词等不能带 B 类可能补语。
不能带 B 类可能补语的形容词有以下两类：
A。某些口语中不常用的，如＂肮脏＂、＂错误＂、＂丑陋＂、＂细腻＂、＂衰败＂、＂高大＂等。
B。非谓形容词，如＂男＂、＂女＂、＂雌＂、＂雄＂，＂正＂、＂副＂、＂横＂、＂竖＂、＂夹＂、＂大型＂、＂初级＂、＂多项＂、＂个别＂、＂共同＂、＂主要＂、＂新生＂、＂慢性＂、＂新式＂、＂四方＂、＂万能＂、＂天然＂、＂人为＂、＂袖珍＂、＂高频＂等。
２。B 类可能补语中间不能插入程度副词。
I think "到" is more like a colloquial expression that is the equivalent of "了", and its usage depends on regions. So there is no real strict rule saying it can only be used in conjunction with subject/object whatsoever.
The title of this thread relates to the use of 到 as a complement.
In Chinese grammar, the word ‘complement’ (補語) is mostly used as the shortened form for verbal complement (動詞補語 or 動補). As the name has suggested, a verbal complement is used only in conjunction with the preceding verb, and is thus connected neither to the subject nor to the object.
To be specific, there are 4 main types of verbal complements: resultative (結果補語), directional (趨向補語), potential (可能補語), and extent (程度補語). In Chinese grammar, 到 can be used only as a resultative complement. In the examples of 去到, 找都, 收到 etc, the first characters are the head verbs, indicating the process of an action; the second character 到 is the resultative complement, indicating the result of an action. The two characters together are called a resultative verb (結果動詞).
In Cantonese grammar, 到 can function as an extent complement (as well as a resultative complement), e.g. 行到好癐, 笑到轆地 ‘laugh oneself silly’. But 到 can’t be used this way in Chinese grammar.
As regards the phrase 买不到 as supplied by the OP, 买 is the head verb, 到 is the resultative complement, and 不 is the potential complement (indicating the inability of completing an action).
