For example, 'She was a good teacher' ? Any and all translations I get are '她是好老师'. Is there anyway to indicate past tense explicitly?
You have to remember, unlike in English, we do not have past tense for verbs in Chinese grammar. Both "is" and "was" is written as "是" in Chinese.
"是" in "她是好老师" could be "is" or "was". We really don't care which, because it is presumed readers can find out the sentence is in past or present tense by looking into the context.
"她以前是个好老师"(she was a good teacher before)
"她曾经是个好老师" (she was a good teacher once)
"她三年前是个好老师"(she was a good teacher three years ago)
All three example sentences above clearly indicated the verb "是" is in past tense.
If no time reference is found in the context, then all verbs are in present tense by default.
When you are reading a historical novel, even most individual sentences in it didn't have time reference, you should still know it is in past tense; If you are reading an instructional manual, you know the text is written in present tense entirely.
The answer to "How do you specify past tense for 是 " should be "there's no need"
This is a frequent type of question, perhaps someone can ask a more general question that would allow a more generally useful answer. As a step toward that, I'll try to raise a couple of general points here.
First, past tense is not a vocabulary item in English. It is a type of verbal inflection. Chinese does not have this type of verbal inflection. Answers here that propose using words like 以前 or 曾經 are using vocabulary items to try and "translate" English past tense into Chinese. This is not a general solution; that is, it will not work for all Chinese verbs in all situations. For example,
is not going to be translated as
So 曾經 is not always an equivalent of past tense.
This is also true here. As someone else noted above, a sentence like
is probably more like to be interpreted as equivalent to English
She used to be a good teacher.
This is probably not what the OP meant when asking how to say "She was a good teacher" in Chinese.
This brings in the second part of the problem in answering this type of question. When people ask things like how to say "She was a good teacher", they usually do so without stopping to think that such English sentence may have more than one meaning or interpretation. In many cases, Chinese equivalents may be radically different for the different English meanings.
For example, we might have the following context:
My sixth grade math teacher was Mrs. Li. She was a great teacher.
In this case, one very natural Chinese translation would be.
It would actually be somewhat odd to translate as
In this case, the demand that English "tense" be directly reflected in the Chinese is a mistake. Chinese simply doesn't use words like 曾經 or 以前 in this way.
In such cases, asking questions like "How do you say 'She was a good teacher'", without also specifying the context, can often lead to confusing or contradictory answers.
As others mentioned, The paste tense for "是" is usually "曾经" or "过去". But here might be a little different. "She was a good teacher" implies that she has died now. So it might be more clear to say "她生前是个好老师".
If someone say "她曾经/过去是个好老师", it might be more similar to "She used to be a good teacher", and might imply (but not always imply) that she is not a good teacher now.
'She was a good teacher' 她曾经是位好老师
曾经 puts this firmly in the past.
A lot of the time you can use 了 or 过 to indicate the past. Here is an explanation thereof from another forum. I thought it sums up their usage quite well.
“了” 和 “过” 的用法是非常相似，希望你能慢慢体会，多多练习。(ง •_•)ง,加油
Chinese is pretty straightforward, we can just say
以前 (in the past) + whatever and we have 'was whatever'.
before; formerly; previously
以前我家生活不太好。 Yǐqián wǒ jiā shēnghuó bù tài hǎo.
In the past, life was hard for my family.
If it helps, the above sentence you can even think of it without "In the past," and just think of it as "life was hard for my family."
A Chinese-English Dictionary
以前他当过理发员。 Yǐqián tā dāng guò lǐfàyuán.
He used to be a barber.
If you want to use 是 you can easily say:
So for your sentence She was a good teacher you can simply say:
曾 or 曾经 on the other hand only means
This form of "was" only really works if said person is not that way anymore - not dead! If the person is dead, it's best to just refer to them in the presets tense, the context will be clear enough for people to know.