The section called Key Concepts in the ELT Journal has quite short articles on a wide range of language acquisition concepts.
Each of these links opens a pdf of the article.
Learner training (ELT Journal 47:1)
Learner strategies (ELT Journal 47:1)
Fluency (ELT Journal 47:3)
Project work (ELT Journal 47:3)
Pragmatics (ELT Journal 48:1)
Scaffolding (ELT Journal 48:1)
Task-based learning and pedagogy (ELT Journal 53:1)
Feedback (ELT Journal 48:3)
Register (ELT Journal 48:3)
Universal grammar (ELT Journal 49:2)
Noticing (ELT Journal 50:3)
Schemas (ELT Journal 51:1)
Classroom research (ELT Journal 51:2)
Anaphora (ELT Journal 51:4)
Deductive vs. inductive language learning (ELT Journal 52:1)
Task (ELT Journal 52:3)
Genre (ELT Journal 53:2)
Evaluation (ELT Journal 54:2)
Lexical Chunks (ELT Journal 54:4)
Teachers' beliefs (ELT Journal 55:2)
Language-related episodes (ELT Journal 55:3)
'Focus on form' vs 'Focus on forms' (ELT Journal 56:3)
Observation (ELT Journal 57:2)
Loop input (ELT Journal 57:3)
Discourse communities (ELT Journal 57:4)
Globalization and language teaching (ELT Journal 58:1)
The apprenticeship of observation (ELT Journal 58:3)
Washback and impact (ELT Journal 59:2)
English as a lingua franca (ELT Journal 59:4)
The Common European Framework (ELT Journal 60:2)
Native-speakerism (ELT Journal 60:4)
Processing instruction (ELT Journal 61:2)
Motivation in ELT (ELT Journal 61:4)
Learner self-beliefs (ELT Journal 62:2)
Learner autonomy (ELT Journal 62:4)
Age and the critical period hypothesis (ELT Journal 63:2)
Innovation in ELT (ELT Journal 63:4)
Expertise in language learning and teaching (ELT Journal 64:2)