Broadly, you can consider them as synonyms. This is to make us understand that when you are talking about something that happened in 'past', grammatically you are talking in 'past tense'. And, it is simply past tense without describing any mood or completion of a particular event (or else it then starts having other types of past i.e. past perfect etc.).
Wikipedia says...
The past tense is a grammatical tense whose principal function is to place an action or situation in past time. In languages which have a past tense, it thus provides a grammatical means of indicating that the event being referred to took place in the past. Examples of verbs in the past tense include the English verbs sang, went and was.
Having this said, past tense is actually an umbrella term which covers many things in English grammar. Under past tense, there could be many other types such as past perfect, past participle and so on.
Past tense is generally considered as a Simple Past if you have not specified anything about it.
About.com describes it as a synonym by putting it into brackets. There are also other terms for this mentioned on the same page.
The simple past tense (also known as the past simple or preterite) of regular verbs is marked by the ending -d, -ed, or -t. Irregular verbs have a variety of endings. The simple past is not accompanied by helping verbs.
Both phrases are valid, but they both mean slightly different things. The phrase "impossible is nothing" evolved from "nothing is impossible".
Consider the following dialogue:
Person A: We can't do X - it would be impossible!
Person B: Nothing is impossible.
In this scenario, person A claims that doing X is not possible, but person B claims that this is not true. "Nothing is impossible" is a rejection of the claim that X cannot be done, by suggesting that problems abandoned for being "impossible" are typically "hard" rather than "impossible".
The phrase "impossible is nothing" is a very modern extension of this phrase into a boast, as shown below:
Person B: Nothing is impossible.
Person C: For me, impossible is nothing.
In this scenario, person B claims as before that many things people think are impossible are merely hard. But person C is saying that such problems are nothing - that is, trivial - for him.
So in summary, "Nothing is impossible" is a motivational phrase rejecting claims that something is "impossible" by claiming that it is only "difficult" rather than "impossible".
But "Impossible is nothing" is a boast - a claim that the speaker is able to easily perform feats that other people would think impossible to achieve.
Best Answer
In the second pattern, the subject complement would be the past participle of a transitive verb.
The past participle of to lay is laid.
Someone lays present the paper on the desk each morning.
Someone laid past the paper on the desk.
Someone has laid present perfect the paper on the desk.