"bas" is an adjective. If you are talking about a table, you will say "elle est basse" (une table), if a stool "il est bas" (un tabouret).
"tard" is an adverb. If you mean "he is late", you will have to say "il est en retard", because the impersonal "il est tard" means "it is late".
For once, I have to question your input Sitesurf. I checked several French-English dictionaries, and it lists "bas" as meaning base as in "contemptible" and "vile". I figured this was the root, as to be "low (now "lowly") used to be synonymous with this form as well. However, this meaning is fairly archaic, and I've only seen it in poetry and old texts.
In English Canada, 'low' is usually a feeling. You can say "He is feeling low" meaning 'he is depressed'. You can also say "he is feeling down" to say the same thing. 'low' could also mean a person has low morals or no scruples but you won't hear that often. Usually 'low' refers to feeling depressed.