"We drank tea."
Translation:हमने चाय पी।
Also, if the direct object is in the oblique case, just put the verb in the third person singular masculine (the default conjugation). e.g.
मैंने तारे देखे। - conjugate according to the direct object.
मैंने आपको देखा। - आपको is in oblique case, so verb defaults to 3rd sing masc.
मैं सोया/सोयी। - verb is intransitive, so conjugate according to the subject.
हमने देखा। - This is a bit complicated. देखना is a transitive verb. The sentence should have a direct object but it doesn't (this is mostly done in informal speech, in formal speech it is better to provide something like उसको/कुछ). In such cases, again default to 3rd masc sing. I have also heard people speak as though देखना is intransitive in such cases and say "हम देखे," but that sounds ungrammatical to me.
For the simple past tense of पीना, पिया is the masculine singular form, पी is the feminine form and पिए is the masculine plural form.
Also, in the simple past tense, the verb पीना conjugates with its object(s) rather than the subject.
मैंने पानी पिया - I drank water - because पानी is masculine
मैंने दूध और पानी दोनों पिए। - I drank both water and milk - because the object (दूध और पानी ) is plural masculine
मैंने लस्सी पी - I drank lassi (an Indian yoghurt based drink) - because लस्सी is feminine
Unfortunately, there is no foolproof way of knowing the gender of a noun just by looking at it because it is based on many different factors such as word form, etymology, meaning etc.
You can sometimes guess the gender based on things like word ending (words ending with ा are more likely to be male, those ending in ी are more likely to be female etc) or the category of the word (country names are usually male, rivers are usually female etc) but there are a lot of exceptions to each of these rules. The best way is that whenever you encounter a new Hindi word, you learn it with its gender. That is, instead of learning कुत्ता='dog', बिल्ली='cat', you learn मेरा कुत्ता='my dog', मेरी बिल्ली='my cat'.